Let’s do the Landau shuffle

Here’s a shuffle I’ve not seen before:

  1. Take an ordinary deck of 52 cards and place them, face up, in the following pattern:
    Going from the top of the deck to the bottom, placing cards down left-to-right, put 13 cards in the top row:
    \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\
    11 cards in the next row:
    \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\
    then 9 cards in the next row:
    \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\
    then 7 cards in the next row:
    \Box\ \Box\ \Box\ \Box\ \Box\ \Box\ \Box\
    then 5 cards in the next row:
    \Box\ \Box\ \Box\ \Box\ \Box\
    then 3 cards in the next row:
    \Box\ \Box\ \Box\
    and finally, the remaining 4 cards in the last row:
    \Box\ \Box\ \Box\ \Box\
  2. Now, take the left-most card in each row and move it to the right of the others (effectively, this is a cyclic shift of that row of cards to the left).
  3. Finally, reassemble the deck by reversing the order of the placement.

In memory of the great German mathematician Edmund Landau (1877-1938, see also this bio), I call this the Landau shuffle. As with any card shuffle, this shuffle permutes the original ordering of the cards. To restore the deck to it’s original ordering you must perform this shuffle exactly 180180 times. (By the way, this is called the order of the shuffle.) Yes, one hundred eighty thousand, one hundred and eighty times. Moreover, no other shuffle than this Landau shuffle will require more repetitions to restore the deck. So, in some sense, the Landau shuffle is the shuffle that most effectively rearranges the cards.

Now suppose we have a deck of n (distictly labeled) cards, where n>2 is an integer. The collection of all possible shuffles, or permutations, of this deck is denoted S_n and called the symmetric group. The above discussion leads naturally to the following question(s).

Question: What is the largest possible order of a shuffle of this deck (and how do you construct it)?

This requires a tiny bit of group theory. You only need to know that any permutation of n symbols (such as the card deck above) can be decomposed into a composition or product) of disjoint cycles. To compute the order of an element g \in S_n, write that element g in disjoint cycle notation. Denote the lengths of the disjoint cycles occurring in g as k_1, k_2, \dots , k_m, where k_i>0 are integers forming a partition of n: n = k_1 + k_2 +\dots + k_m. Then the order of g is known to be given by order(g) = LCM(k_1, k_2, ...., k_m), where LCM denotes the least common multiple.

The Landau function g(n) is the function that returns the maximum possible order of an element g \in S_n. Landau introduced this function in a 1903 paper where he also proved the asymptotic relation \lim_{n\to \infty} \frac{\ln(g(n))}{\sqrt{n\ln(n)}}=1.

Example: If n=52 then note 52 = 13+11+9+7+5+3+4 and that LCM(13,11,9,77,5,3,4)=180180.

Oddly, my favorite mathematical software program SageMath does not have an implementation of the Landau function, so we end with some SageMath code.

def landau_function(n):
L = Partitions(n).list()
lcms = [lcm(P) for P in L]
return max(lcms)

Here is an example (the time is included to show this took about 2 seconds on my rather old mac laptop):

sage: time landau_function(52)
CPU times: user 1.91 s, sys: 56.1 ms, total: 1.97 s
Wall time: 1.97 s

A mathematical card trick

If you search hard enough on the internet you’ll discover a pamphlet from the 1898 by Si Stebbins entitled “Card tricks and the way they are performed” (which I’ll denote by [S98] for simplicity). In it you’ll find the “Si Stebbins system” which he claims is entirely his own invention. I’m no magician, by from what I can dig up on Magicpedia, Si Stebbins’ real name is William Henry Coffrin (May 4 1867 — October 12 1950), born in Claremont New Hampshire. The system presented below was taught to Si by a Syrian magician named Selim Cid that Si sometimes worked with. However, this system below seems to have been known by Italian card magicians in the late 1500’s. In any case, this blog post is devoted to discussing parts of the pamphlet [S98] from the mathematical perspective.

In stacking the cards (face down) put the 6 of Hearts 6 \heartsuit first, the 9 of Spades 9 \spadesuit next (so it is below the 6 \heartsuit in the deck), and so on to the end, reading across left to right as indicted in the table below (BTW, the pamphlet [S98] uses the reversed ordering.) My guess is that with this ordering of the deck — spacing the cards 3 apart — it still looks random at first glance.

Hearts \heartsuitSpades \spadesuitDiamonds \diamondsuitClubs \clubsuit
Si Stebbins’ System

Next, I’ll present a more mathematical version of this system to illustrate it’s connections with group theory.

We follow the ordering suggested by the mnemonic CHaSeD, we identify the suits with numbers as follows: Clubs is 0, Hearts is 1, Spades is 2 and Diamonds is 3. Therefore, the suits may be identified with the additive group of integers (mod 4), namely: {\bf{Z}}/4{\bf{Z}} = \{ 0,1,2,3 \}.

For the ranks, identify King with 0, Ace with 1, 2 with 2, \dots, 10 with 10, Jack with 11, Queen with 12. Therefore, the ranks may be identified with the additive group of integers (mod 13), namely: {\bf{Z}}/13{\bf{Z}}=\{ 0,1,2,\dots 12\}.

Rearranging the columns slightly, we have the following table, equivalent to the one above.

Mathematical version of the Si Stebbins Stack

In this way, we identify the card deck with the abelian group

G = {\bf{Z}}/4{\bf{Z}} \times {\bf{Z}}/13{\bf{Z}} .

For example, if you spot the 2 \clubsuit then you know that 13 cards later (and If you reach the end of the deck, continue counting with the top card) is the 2 \heartsuit, 13 cards after that is the 2 \spadesuit, and 13 cards after that is the 2 \diamondsuit.

Here are some rules this system satisfies:

Rule 1 “Shuffling”: Never riff shuffle or mix the cards. Instead, split the deck in two, the “bottom half” as the left stack and the “top half” as the right stack. Take the left stack and place it on the right one. This has the effect of rotating the deck but preserving the ordering. Do this as many times as you like. Mathematically, each such cut adds an element of the group G to each element of the deck. Some people call this a “false shuffle” of “false cut.”

Rule 2 “Rank position”: The corresponding ranks of successive cards in the deck differs by 3.

Rule 3 “Suit position”: Every card of the same denomination is 13 cards apart and runs in the same order of suits as in the CHaSeD mnemonic, namely, Clubs \clubsuit, Hearts \heartsuit, Spades \spadesuit, Diamonds \diamondsuit.

At least, we can give a few simple card tricks based on this system.

Trick 1: A player picks a card from the deck, keeps it hidden from you. You name that card.

This trick can be set up in more than one way. For example, you can either

(a) spread the cards out behind the back in such a manner that when the card is drawn you can separate the deck at that point bringing the two parts in front of you, say a “top” and a “bottom” stack, or

(b) give the deck to the player, let them pick a card at random, which separates the deck into two stacks, say a “top” and a “bottom” stack, and have the player return the stacks separately.

You know that the card the player has drawn is the card following the bottom card of the top stack. If the card on the bottom of the top stack is denoted (a,\alpha) \in G and the card drawn is (b,\beta) then

b \equiv a+3 \pmod{13}, \ \ \ \ \ \ \beta \equiv \alpha +1 \pmod{4}.

For example, a player draws a card and you find that the bottom card is the 9 \diamondsuit. What is the card the player picked?

solution: Use the first congruence listed: add 3 to 9, which is 12 or the Queen. Use the second congruence listed: add one to Diamond \diamondsuit (which is 3) to get 0 \pmod 4 (which is Clubs \clubsuit). The card drawn is the Q \clubsuit.

Trick 2: Run through the deck of cards (face down) one at a time until the player tells you to stop. Name the card you were asked to stop on.

Place cards behind the back first taking notice what the bottom card is. To get the top card, add 3 to the rank of the bottom card, add 1 to the suit of the bottom card. As you run through the deck you silently say the name of the next card (adding 3 to the rank and 1 to the suit each time). Therefore, you know the card you are asked to stop on, as you are naming them to yourself as you go along.

Dodecahedral Faces of M12


Dodecahedral Faces of M12


by Ann Luers Casey


This post constitutes part of the math honors thesis written in spring 1997 at the USNA, advised by David Joyner. It is in the public domain.

Groups are objects in mathematics that measure symmetry in nature. A group is a set with a binary operation that has an inverse, an identity and is associative. For example, a clock has 12-fold symmetry. A more unusual group is a sporadic, non-abelian simple group. It can be very interesting to look more closely at such a group that arises naturally. One such group is M12. This post explores two different ways of creating M12 and then looks at twelve different ways M12 appears in mathematics, hence the pun the “dodecahedral faces” in the title. Specifically, this post relates M12 to the Mongean shuffle, hexads of a Steiner system, Golay codes, the Hadamard matrix of order 12, 5-transitivity, presentations, crossing the Rubicon, the minimog, the kitten, mathematical blackjack, sporadic groups, and the stabilizer in M24 of a dodecad.


Homomorphism: Let G1, G2 be groups with *1 denoting the group operation for G1 and *2 the group operation for G2. A function f : G1–>G2 is a homomorphism if and only if for all a,b, in Gwe have

f(a *1 b) = f(a) *2 f(b).

Isomorphism: If a homomorphism is bijective, then it is called an

Automorphism: An isomorphism from a group G to itself is an automorphism.


  • Let Fq denote the finite field with q elements, q is a power of a prime.
  • Z = the invertible scalar 2×2 matrices with entries in Fqx.
  • Let PGL2(Fq) = GL2(Fq)/Z = {A*Z | A is in GL2(Fq)}, with multiplication given by
    (A*Z)(B*Z) = (A*B)Z. This is the projective linear group over Fq.
  • LF(Fq) is the group of linear fractional transformations x–>(ax+b)/(cx+d).

Claim: There is a group theoretic isomorphism between PGL2(Fq) and LF(Fq). (See [11], Theorem 9.47 for a proof.)

Claim: LF(Fq) acts 3-transitively on the set P1(Fq) (q>3). I.e., one can send any triple to any other triple in P1(Fq) by using a suitable linear fractional transformation. (See [11], Theorem 9.48 for a proof.)


PSL2(Fq) = < x–>x+1, x–>kx, x–>-1/x>, where k is any element in Fqthat generates the multiplicative group of squares.

For a proof, see [12], ch 10, section 1.

One way to construct the Mathieu group M12 is the following, accredited to Conway.

M12 = < PSL2(F11), (2 10)(3 4)(5 9)(6 7) >.More explicitly, let

  • f1 be a cyclic permutation = x–> x+1 = (0,1,2,…,10)(inf).
  • f2 = x–>kx = (0)(1 3 9 5 4)(2 6 7 10 8)(inf) when k=3.
  • f3 = x–>-1/x = (0 inf)(1 10)(2 5)(3 7)(4 8)(6 9).
  • f4 = (2 10)(3 4)(5 9)(6 7).

Then M12 = < f1, f2, f3, f4 >. Therefore, M12 is a subgroup of the symmetric group on 12
symbols, namely inf, 0, 1, …, 10.

Another way to construct M12 is given later under 5-transitivity.

There are many occurrences of M12 in mathematics, but here I will list and explain twelve of them:

  1. Mongean Shuffle
  2. Steiner Hexad
  3. Golay Code
  4. Hadamard Matrices
  5. 5- Transitivity
  6. Presentations
  7. Crossing the Rubicon
  8. <a href="m_12.htm#M12 and the Minimog”>M12 and the Minimog
  9. Kitten
  10. Mathematical Blackjack or Mathieu’s 21
  11. Sporadic Groups
  12. <a href="m_12.htm#Stabilizer in M24 of a dodecad”>Stabilizer in M24 of a dodecad

    1. Mongean Shuffle


    The Mongean shuffle concerns a deck of twelve cards, labeled 0 through 11. The permutation

    r(t) = 11-t

    reverses the cards around. The permutation

    s(t) = min(2t,23-2t)

    is called the Mongean Shuffle. The permutation group M12 is generated by r and s: M12 = < r,s >, as a subgroup of S12. (See [12], Chap. 11, Sec. 17 or [18])

    2. Steiner Hexad


    Jacob Steiner (1796-1863) was a Swiss mathematician specializing in projective goemetry. (It is said that he did not learn to read or write until the age of 14 and only started attending school at the age of 18.) The origins of “Steiner systems” are rooted in problems of plane geometry.

    Let T be a given set with n elements. Then the Steiner system S(k,m,n) is a collection S = {S1, … ,Sr} of subsets of T such that

    • |Si| = m,
    • For any subset R in T with |R| = k there is a unique i, 1<=i<=n such that R is contained in Si. |S(k,m,n)| = (n choose k)/(m choose k).

    If any set H has cardinality 6 (respectively 8, 12) then H is called a hexad, (respectively octad, dodecad.)

    Let’s look at the Steiner System S(5,6,12) and M12. We want to construct the Steiner system S(5,6,12) using the projective line P1(F11). To define the hexads in the Steiner system, denote

    • the projective line over F11 by P1(F11)={inf,0,1,…,10}.
    • Q = {0,1,3,4,5,9}=the quadratic residues union 0
    • G = PSL2(F11)
    • S = set of all images of Q under G. (Each element g in G will send Q to a subset of P1(F11). )

    There are always six elements in such a hexad. There are 132 such hexads. If I know five of the elements in a hexad of S, then the sixth element is uniquely determined. Therefore S is a Steiner system of type (5,6,12).

    M12 sends a hexad in a Steiner system to another hexad in a Steiner system. In fact, the automorphism group of a Steiner system of type (5,6,12) is isomorphic to M12.

    (For a proof, see [11], Theorem 9.78.)

    The hexads of S form a Steiner system of type (5,6,12), so

    M12 = < g in S12 | g(s) belongs to S, for all s in S > .

    In other words, M12 is the subgroup stabilizing S. The hexads support the weight six words of the Golay code, defined next. (For a proof, see  [6].)

    3. Golay Code


    ” The Golay code is probably the most important of all codes for both practical and theoretical reasons.” ([17], pg. 64)

    M. J. E. Golay (1902-1989) was a Swiss physicist known for his work in infrared spectroscopy among other things. He was one of the founding fathers of coding theory, discovering GC24 in 1949 and GC12 in 1954.

    A code C is a vector subspace of (Fq)for some n >=1 and some prime power q =pk.
    An automorphism of C is a vector space isomorphism, f:C–>C.

    If w is a code word in Fqn, n>1, then the number of non-zero coordinates of w is called the weight w, denoted by wt(w). A cyclic code is a code which has the property that whenever (c0,c1,…,cn-1) is a code word then so is (cn-1,c0,…,cn-2).
    If c=(c0,c1,…,cn-1) is a code word in a cyclic code C then we can associate to it a polynomial g_c(x)=c0 + c1x + … + cn-1xn-1. It turns out that there is a unique monic polynomial with coefficients in Fq

    of degree >1 which divides all such polynomials g_c(x). This polynomial is called
    a generator polynomial for C, denoted g(x).

    Let n be a positive integer relatively prime to q and let alpha be a primitive n-th root of unity. Each generator g of a cyclic code C of length n has a factorization of the form g(x) = (x-alphak1)… (x-alphakr), where {k1,…,kr} are in {0,…,n-1} [17]. The numbers alphaki, 1≤ i≤ r, are called the zeros of the code C.

    If p and n are distinct primes and p is a square mod n, then the quadratic residue code of length n over Fp is the cyclic code whose generator polynomial has zeros
    {alphak | k is a square mod n} [17]. The ternary Golary code GC11 is the quadratic
    residue code of length 11 over F3.

    The ternary Golay code GC12 is the quadratic residue code of length 12 over F3 obtained by appending onto GC11 a zero-sum check digit [12].

    There is a normal subgroup N of Aut(GC12) of order 2 such that Aut(GC12)/N is isomorphic to M12. M12 is a quotient of Aut(GC12) by a subgroup or order 2. In other words, M12 fits into the following short exact sequence:


    Where i is the embedding and N in Aut(GC12) is a subgroup of order 2. See [6].

    4. Hadamard Matrices


    Jacques Hadamard (1865-1963) was a French mathematician who did important work in analytic number theory. He also wrote a popular book “The psychology in invention in the mathematical field” (1945).

    A Hadamard matrix is any n x n matrix with a +1 or -1 in every entry such that the absolute value of the determinant is equal to nn/2.

    An example of a Hadamard matrix is the Paley-Hadamard matrix. Let p be a prime of the form 4N-1, p > 3. A Paley-Hadamard matrix has order p+1 and has only +1’s and -1’s as entries. The columns and rows are indexed as (inf,0,1,2,…,p-1). The infinity row and the infinity column are all +1’s. The zero row is -1 at the 0th column and at the columns that are quadratic non-residues mod p; the zero row is +1 elsewhere. The remaining p-1 rows are cyclic shifts of the finite part of the second row. For further details, see for example [14].

    When p = 11 this construction yields a 12×12 Hadamard matrix.

    Given two Hadamard Matrices A, B we call them left-equivalent if there is an nxn signed permutation matrix P such that PA = B.

    The set {P nxn signed permutation matrix| AP is left equivalent to A} is called the automorphism group of A. In other words, a matrix is an automorphism of the Hadamard matrix, if it is a nxn monomial matrix with entries in {0,+1,-1} and when it is multiplies the Hadamard matrix on the right, only the rows may be permuted, with a sign change in some rows allowed.

    Two nxn Hadamard matrices A, B are called equivalent if there are nxn signed permutation matrices P1, P2 such that A = P1 *B *P2.

    All 12×12 Hadamard matrices are equivalent ([13][16] pg. 24). The group of automorphisms of any 12×12 Hadamard matrix is isomorphic to the Mathieu group M12 ([14] pg 99).


    5. 5-Transitivity


    Emile Mathieu (1835-1890) was a mathematical physicist known for his solution to the vibrations of an elliptical membrane.

    The fact that M12 acts 5-transitively on a set with 12 elements is due to E. Mathieu who proved the result in 1861. (Some history may be found in [15].)

    There are only a finite number of types of 5-transitive groups, namely Sn (n>=5), An (n>=7), M12 and M24. (For a proof, see [11])

    Let G act on a set X via phi : G–>SX. G is k-transitive if for each pair of ordered k-tuples (x1, x2,…,xk), (y1,y2,…,yk), all xi and yi elements belonging to X, there exists a g in G such that yi = phi(g)(xi) for each i in {1,2,…,k}.

    M12 can also be constructed as in Rotman [11], using transitive extensions, as follows (this construction appears to be due originally to Witt). Let fa,b,c,d(x)=(ax+b)/(cx+d), let

    M10 = < fa,b,c,d, fa’,b’,c’,d’ |ad-bc is in Fqx, a’d’-b’c’ is not in Fqx >,

    q = 9.

    pi = generator of F9x, so that F9x = < pi> = C8.

    Using Thm. 9.51 from Rotman, we can create a transitive extension of M10. Let omega be a new symbol and define

    M11 = < M10, h| h = (inf, omega)(pi, pi2)(pi3,pi7) (pi5,pi6)>.

    Let P1(F9) = {inf, 0, 1, pi, pi2, … , pi7}. Then M11 is four transitive on Y0 = P1(F9) union {omega}, by Thm 9.51.

    Again using Thm. 9.51, we can create a transitive extension of M11. Let sigma be a new symbol and define

    M12 = < M11, k>, where k = (omega, sig)(pi,pi3) (pi2,pi6)(pi5,pi7). M12 is 5-transitive on Y1 = Y0 union {sig}, by Th. 9.51.

    Now that we constructed a particular group that is 5-transitive on a particular set with 12 elements, what happens if we have a group that is isomorphic to that group? Is this new group also 5-transitive?

    Let G be a subgroup of S12 isomorphic to the Mathieu group M12. Such a group was constructed in Section 1.

    Lemma: There is an action of G on the set {1,2,…,12} which is 5-transitive.

    proof: Let Sig : G –> M12 be an isomorphism. Define g(i) = Sig(g)(i), where i = {1,2,…,12}, g is in G. This is an action since Sig is an isomorphism. Sig-1(h)(i) = h(i) for all g in M12, i in Y1. Using some h in M12, any i1,…,i5

    in Y1 can be sent to any j1,…,j5 in Y1. That is, there exists an h in M12 such that h(ik) = jk, k= 1,…,5 since M12 is 5-transitive. Therefore, Sig-1(h)(ik) = jk = g(ik). This action is 5-transitive. QED

    In fact, the following uniqueness result holds.

    Theorem: If G and G’ are subgroups of S12 isomorphic to M12 then they are conjugate in S12.

    (This may be found in [7], pg 211.)

    6. Presentations


    The presentation of M12 will be shown later, but first I will define a presentation.

    Let G = < x1,…,xn | R1(x1,…xn) = 1, …, Rm(x1,…,xn) = 1> be the smallest group generated by x1,…,xn satisfying the relations R1,…Rm. Then we say G has presentation with generators x1,…,xn and relations R1(x1,…xn) = 1, …, Rm(x1,…,xn) = 1.

    Example: Let a = (1,2,…,n), so a is an n-cycle. Let Cn be the cyclic group, Cn = < a > =
    {1,a,…,an-1}. Then Cn has presentation < x | xn=1 > = all words in x, where x satisfies xn.=1 In fact, < x | xn = 1 > is isomorphic to < a >. Indeed, the isomorphism
    < x | xn = 1 > –> < a > is denoted by xk –> ak, 0 <= k <= n-1. Two things are needed for a presentation:

    • generators, in this case x, and
    • relations, in this case xn = 1.

    Example: Let G be a group generated by a,b with the following relations; a2 = 1, b2 = 1, (ab)2 = 1:

    G = < a,b | a2 = 1, b2 = 1, (ab)2 = 1 > = {1,a,b,ab}.

    This is a non-cyclic group of order 4.

    Two presentations of M12 are as follows:

    M12 = < A,B,C,D | A11 = B5 = C2 = D2 = (BC)2 = (BD)2 = (AC)= (AD)3 = (DCB)2 = 1, AB =A3 >

    = < A,C,D | A11 = C2 = D2 = (AC)3 = (AD)3 = (CD)10 = 1, A2(CD)2A = (CD)2 >.

    In the first presentation above, AB = B-1AB. These are found in [6] and Chap. 10 Sec. 1.6 [12].

    7. Crossing the Rubicon


    The Rubicon is the nick-name for the Rubik icosahedron, made by slicing the icosahedron in half for each pair of antipodal vertices. Each vertex can be rotated by 2*pi/5 radians, affecting the vertices in that half of the Rubicon, creating a shape with 12 vertices, and six slices. The Rubicon and M12 are closely related by specific moves on the Rubicon.

    Let f1, f2, …,f12 denote the basic moves of the Rubicon, or a 2*pi/5 radians turn of the sub-pentagon about each vertex. Then according to Conway,

    M12 = < x*y-1 | x,y are elements of {f1, f2, …,f12 } >.

    Actually, if a twist-untwist move, x*y-1, as above, is called a cross of the Rubicon, then M12 is generated by the crosses of the Rubicon! ([1], Chap. 11 Sec. 19 of [12])

    <a name="M12 and the Minimog”>

    8. M12 and the Minimog


    Using the Minimog and C4 (defined below), I want to construct the Golay code GC12.

    The tetracode C4 consists of 9 words over F3:

      0 000,     0 +++,    0 ---,         where 0=0, +=1, and -=2 all mod 3.
      + 0+-,     + +-0,    + -0+,
      - 0-+,     - +0-,    - -+0.

    Each (a,b,c,d) in C4 defines a linear function f : F3 –> F3, where f(x) = ax+b, f(0) = b, f(1) = f(+) = c, f(2) = f(-) = d, and a is the “slope” of f. This implies a + b = c (mod 3), b – a = d (mod 3).

    Minimog: A 4×3 array whose rows are labeled 0,+,-, that construct the Golay code in such a way that both signed and unsigned hexads are easily recognized.

    A col is a word of length 12, weight 3 with a “+” in all entries of any one column and a “0” everywhere else. A tet is a word of length 12, weight 4 who has “+” entries in a pattern such that the row names form a tetracode word, and “0” entires elsewhere. For example,

                 _________          _________
                 | |+    |          | |+    |
                 | |+    |          |+|  +  |
                 | |+    |          | |    +| 
                 ---------          ---------               
                 "col"              "tet"

    The above “col” has “+” entries in all entries of column 2, and “0” entries elsewhere.
    The above “tet” has a “+” entry in each column. The row names of each “+” entry are +, 1, +, – respectively. When put together, + 0+- is one of the nine tetracode words.

    Lemma: Each word belongs to the ternary Golay Code GC12 if and only if

    • sum of each column = -(sum row0)
    • row+ – row is one of the tetracode words.

    This may be found in [4].


    |+|+ + +|      col sums: ----      row+ - row-: --+0
    |0|0 + -|      row0 sum: + = -(sum of each col)
    |+|+ 0 -|

    How do I construct a Golay code word using cols and tets? By the Lemma above, there are four such combinations of cols and tets that are Golay code words. These are: col – col, col + tet, tet – tet, col + col – tet.


      col-col         col+tet      tet-tet       col+col-tet
     | |+   -|       | |+ +  |    |+|0 + +|      | |- + +|
     | |+   -|       |+|  -  |    |-|  -  |      |-|  0 +|
     | |+   -|       | |  + +|    | |    -|      | |  + 0| 
      ? ? ? ?         + 0 ? -      - ? - +        + 0 + -

    “Odd-Man-Out”: The rows are labeled 0,+,-, resp.. If there is only one entry in a column then the label of the corresponding row is the Odd Man Out. (The name of the odd man out is that of the corresponding row.) If there is no entry or more than one entry in the column then the odd man out is denoted by “?”.

    For example, in the arrays above, the Odd-Men-Out are written below the individual arrays.

    For the Steiner system S(5,6,12), the minimog is labeled as such:

                                  |0  3 inf  2 |
                                  |5  9  8  10 |
                                  |4  1  6  7  |

    The four combinations of cols and tets above that construct a Golay code word yield all signed hexads. From these signed hexads, if you ignore the sign, there are 132 hexads of the Steiner system S(5,6,12) using the (o, inf, 1) labeling discussed in Section 9 below. There are a total of 265 words of this form, but there are 729 Golay code words total. So, although the above combinations yield all signed hexads, they do not yield all hexads of the Golay code ([12] pg. 321).

    The hexad for the tet-tet according to the S(5,6,12) Minimog above would be (0, inf, 2, 5, 8, 7).

    The rules to obtain each hexad in this Steiner system is discussed in Section 9 below.

    A Steiner system of type (5,6,12) and the Conway-Curtis notation can be obtained from the Minimog. S12 sends the 3×4 minimog array to another 3×4 array. The group M12 is a subgroup of S12 which sends the Minimog array to another array also yielding S(5,6,12) in Conway-Curtis notation.

    9. Kitten


    The kitten is also an interesting facet of the Minimog. Created by R.T. Curtis,
    kittens come from the construction of the Miracle Octal Generator, or MOG, also created by R.T. Curtis. (A description of the MOG would be too far afield for this post, but further information on the MOG can be gotten from [3] or [6].)

    Suppose we want to construct a Steiner system from the set T = {0, 1, …, 10, inf}.
    The kitten places 0, 1, and inf at the corners of a triangle, and then creates a rotational symmetry of triples inside the triangle according to R(y) = 1/(1-y) (as in [2], section 3.1). A kitten looks like:

                                    2     10
                                 5     7      3
                              6     9      4     6
                           2    10     8      2     10
                     0                                    1
                                Curtis' kitten               

    where 0, 1, inf are the points at infinity.

    Another kitten, used to construct a Steiner system from the set T = {0, 1, …, 10, 11} is

                                    10     8
                                 7     2      5
                              9     4     11     9
                          10     8     3      10     8
                     1                                    0
                             Conway-Curtis' kitten

    The corresponding minimog is

                      |  6  |  3  |  0  |  9  |
                      |  5  |  2  |  7  | 10  |
                      |  4  |  1  |  8  | 11  |

    (see Conway [3]).

    The first kitten shown consists of the three points at 0, inf, 1 with an arrangement of points of the plane corresponding to each of them. This correspondence is:

             6 |10 | 3              5 | 7 |3               5 | 7 | 3 
             2 | 7 | 4              6 | 9 |4               9 | 4 | 6 
             5 | 9 | 8              2 |10 |8               8 | 2 |10
            inf-picture             0-picture              1-picture

    A union of two perpendicular lines is called a cross. There are 18 crosses of the kitten:

                    |* * * |* * * |* * * |*     |  *   |    * |
                    |*     |  *   |    * |* * * |* * * |* * * |
                    |*     |  *   |    * |*     |  *   |    * |
                    |*     |  *   |* *   |*     |*   * |    * |
                    |*     |  *   |* *   |  * * |  *   |    * |
                    |* * * |* * * |    * |  * * |*   * |* * * |
                    |*   * |    * |  * * |  *   |  * * |* *   |
                    |*   * |* *   |*     |*   * |  * * |    * |
                    |  *   |* *   |  * * |*   * |*     |* *   |

    A square is a complement of a cross. The 18 squares of a kitten are:

                    |      |      |      |  * * |*   * |* *  |
                    |  * * |*   * |* *   |      |      |     |
                    |  * * |*   * |* *   |* *   |*   * |* *  |
                    |  * * |*   * |    * |  * * |  *   |* *   |
                    |  * * |*   * |    * |*     | *  * |* *   |
                    |      |      |* *   |*     |  *   |      |
                    |  *   |* *   |*     |*   * |*     |    * |
                    |  *   |    * |  * * |  *   |*     |* *   |
                    |*   * |    * |*     |  *   |  * * |    * |

    The rules to obtain a hexad in the {0,1,inf} notation are the following:

    • A union of parallel lines in any picture,
    • {0, 1, inf} union any line,
    • {Two points at infinity} union {square in a picture corresponding to omitted point at infinity},
    • {One point at infinity} union {cross in the corresponding picture at infinity}.

    (See [2])

    M12 is isomorphic to the group of automorphisms of the Steiner system S(5,6,12) in the Conway-Curtis notation.

    10. Mathematical Blackjack or Mathieu’s 21

    Mathematical Blackjack is a card game where six cards from the group {0,1,…,11} are laid out face up on a table. The rules are:

    • each player must swap a card with a card from the remaining six, that is lower than the card on the table;
    • the first player to make the sum of all six cards less than 21 loses.

    According to Conway and Ryba [8, section V, part (d)], the winning strategy of this game is to choose a move which leaves a Steiner hexad from S(5,6,12) in the shuffle
    notation, whose sum is greater than or equal to 21, on the table.

    The shuffle notation for the hexad, used in the Mathematical Blackjack game, is shown below (see also the description in the hexad/blackjack page):

                  8 |10 |3            5 |11 |3            5 |11 |3
                  9 |11 |4            2 | 4 |8            8 | 2 |4 
                  5 | 2 |7            7 | 9 |10           9 |10 |7 
                 0-picture          1-picture          6-picture

    Riddle: Assuming the strategy, player A just made a winning hexad move that will force player B to make the sum under 21 on his next turn. Joe Smith walks up to player B and offers to shuffle all 12 cards while player A isn’t looking, for a fee. Player B grabs at his chance thinking that a random shuffle will let him back in the game. How is it that player B still loses?

    Joe is actually working for Player A. Joe does not shuffle the cards randomly, but instead uses the M12 group generated by r, s (see section 1) to shuffle the cards. Since the M12 group preserves hexads, player A still has a winning game. (He and Joe split the money.)

    11. Sporadic Groups


    A simple group is a group with no normal subgroups except itself and {1}. Most simple groups are from a family such as PSL2(Fp), p>3 or An, n >= 5. However there exist some simple groups outside of such well known families. These are called sporadic simple groups. M12 is a sporadic simple group of order 95,040. The only smaller sporadic group is M11 of order 7,920. (See [10] pg. 211)

    <a name="Stabilizer in M24 of a dodecad”>

    12. Stabilizer in M24 of a dodecad.


    M24 is a sporadic simple group of order 244,823,040 containing M12 as a subgroup. The Steiner system S(5,8,24) is a collection of 8 element subsets, called octads, from a 24 element set X, with the property that any five elements in X determine a unique octad in the system. There are (24 choose 5)/(8 choose 5) = 759 of these octads. M24 is the subgroup of SX which sends the set of octads to itself. Two octads, O1, O2, intersect in a subset of X of order 0,2,4,6 or 8 [14]. If |O1 intersect O2| = 2 then O1 + O2 is order 12. Such a subset of X is called a dodecad. M12 is isomorphic to

    {g in M24 | g(O1 + O2) = (O1 + O2)} = the stablizer of the dodecad O1 + O2.
    (See [6] for details)

    Much more information can be received from the references below or from the hexad/blackjack page.



    1. W. D. Joyner, Mathematics of the Rubik’s Cube (USNA Course notes), 1997.
    2. R. T. Curtis, “The Steiner System S(5,6,12), the Mathieu Group M12 and the ‘Kitten’ ,” Computational Group Theory, Academic Press, London, 1984.
    3. J. H. Conway, “hexacode and Tetracode- MOG and MINIMOG,” Computational Group Theory (ed. Atkinson), Academic Press, London, 1984.
    4. Vera Pless, “Decoding the Golay Code,” Transactions of Information Theory, IEEE, 1986, (pgs 561-567).
    5. R. T. Curtis, “A new Combinatorial approach to M24“, Mathematical Proceeding of the Cambridge Philosophical Society, Vol. 79, 1974.
    6. J. H. Conway, R. T. Curtis, S. P. Norton, R. A. Parker, R. A. Wilson, “M12,”,
      Atlas of Finite Groups, Clarendon Press, Oxford, 1985.
    7. Robinson, A Course in the Theory of Groups, Springer, 1996.
    8. J. H. Conway, N. Sloane, “Lexicographic Codes: Error-Correcting Codes
      from Game Theory,” Transactions on Information Theory, IEEE, 1986.
    9. A .Adler, “The modular Curve X(11) and the Mathieu group M11“,
      Proc. London Math Society 74(1997)1-28.
      See also the paper X(11) and M11.
    10. T. Thompson, From Error-Correcting Codes Through Sphere
      Packings to Simple Groups
      , The Mathematical Association of
      America, 1983.
    11. Rotman, J, Introduction to the Theory of Groups, 4th ed.
      Springer-Verlag, 1995.
    12. J. Conway, N. Sloane, Sphere Packings, Lattices, and Groups,
      Springer-Verlag, 3rd ed., 1999.
    13. B. Kostant, “The Graph of the truncated icosahedron and the
      last letter of Galois.” Notices of the A.M.S. 42(1995)959-
    14. E. Assmus, “On the Automorphism Groups of Paley-Hadamard
      Matrices.” Combinatorial Mathematics and its Applications.
      University of North Carolina Press, 1969, (pgs 98-103).
    15. P. Greenberg, Mathieu Groups, Courant Institute of Math and
      Science, New York University, 1973.
    16. P. Cameron, J. Van Lint, Designs, Graphs, Codes, and Their
      , London Mathematical Society, Cambridge University
      Press, 1991.
    17. F. MacWilliams, N. Sloane, The Theory of Error Correcting
      , North Holland Publishing Company, 1978.
    18. R. Graham, P. Diaconis, W. Kantor, “The Mathematics of
      Perfect Shuffles”, Advanced Applied Math, Vol. 4, 1985, (pgs

    Typed into html by wdj, 4-18-97.
    Corrections 4-27-2001.
    Last updated 2018-06-10.


MINIMOGs and Mathematical blackjack

This is an exposition of some ideas of Conway, Curtis, and Ryba on S(5,6,12) and a card game called mathematical blackjack (which has almost no relation with the usual Blackjack).

Many thanks to Alex Ryba and Andrew Buchanan for helpful discussions on this post.


An m-(sub)set is a (sub)set with m elements. For integers k<m<n, a Steiner system S(k,m,n) is an n-set X and a set S of m-subsets having the property that any k-subset of X is contained in exactly one m-set in S. For example, if X = \{1,2,\dots,12\}, a Steiner system S(5,6,12) is a set of 6-sets, called hexads, with the property that any set of 5 elements of X is contained in (“can be completed to”) exactly one hexad.

Rob Beezer has a nice Sagemath description of S(5,6,12).

If S is a Steiner system of type (5,6,12) in a 12-set X then any element the symmetric group \sigma\in Symm_X\cong S_{12} of X sends S to another Steiner system \sigma(S) of X. It is known that if S and S’ are any two Steiner systems of type (5,6,12) in X then there is a \sigma\in Symm_X such that S'=\sigma(S). In other words, a Steiner system of this type is unique up to relabelings. (This also implies that if one defines M_{12} to be the stabilizer of a fixed Steiner system of type (5,6,12) in X then any two such stabilizer groups, for different Steiner systems in X, must be conjugate in Symm_X. In particular, such a definition is well-defined up to isomorphism.)

Curtis’ kitten


NICOLE SHENTING – Cats Playing Poker Cards

J. Conway and R. Curtis [Cu1] found a relatively simple and elegant way to construct hexads in a particular Steiner system S(5,6,12) using the arithmetical geometry of the projective line over the finite field with 11 elements. This section describes this.

Let \mathbf{P}^1(\mathbf{F}_{11}) =\{\infty,0,1,2,...,9,10\} denote the projective line over the finite field \mathbf{F}_{11} with 11 elements. Let Q=\{0,1,3,4,5,9\} denote the quadratic residues with 0, and let L=\cong PSL(2,\mathbf{F}_{11}), where \alpha(y)=y+1 and \beta(y)=-1/y. Let S=\{\lambda(Q)\ \vert\ \lambda\in L\}.

Lemma 1: S is a Steiner system of type (5,6,12).

The elements of S are known as hexads (in the “modulo 11 labeling”).

 	 	 	 	2	 	10	 	 	 	 
 	 	 	5	 	7	 	3	 	 	 
 	 	6	 	9	 	4	 	6	 	 
 	2	 	10	 	8	 	2	 	10	 
0	 	 	 	 	 	 	 	 	 	1

Curtis’ Kitten.

In any case, the “views” from each of the three “points at infinity” is given in the following tables.

6	10	3
2	7	4
5	9	8
picture at \infty

5	7	3
6	9	4
2	10	8
picture at 0	

5	7	3
9	4	6
8	2	10
picture at 1

Each of these 3\times 3 arrays may be regarded as the plane \mathbf{F}_3^2. The lines of this plane are described by one of the following patterns.

\bullet	\bullet	\bullet
\times	\times	\times
\circ	\circ	\circ	
slope 0	

\bullet	\times	\circ
\bullet	\times	\circ
\bullet	\times	\circ	
slope infinity

\bullet	\times	\circ
\circ	\bullet	\times
\times	\circ	\bullet	
slope -1

\times	\circ	\bullet
\circ	\bullet	\times
\bullet	\times	\circ
slope 1

The union of any two perpendicular lines is called a cross. There are 18 crosses. The complement of a cross in \mathbf{F}_3^2 is called a square. Of course there are also 18 squares. The hexads are

  1. \{0,1,\infty\}\cup \{{\rm any\ line}\},
  2. the union of any two (distinct) parallel lines in the same picture,
  3. one “point at infinity” union a cross in the corresponding picture,
  4. two “points at infinity” union a square in the picture corresponding to the omitted point at infinity.

Lemma 2 (Curtis [Cu1]) There are 132 such hexads (12 of type 1, 12 of type 2, 54 of type 3, and 54 of type 4). They form a Steiner system of type $(5,6,12)$.

The MINIMOG description

Following Curtis’ description [Cu2] of a Steiner system S(5,8,24) using a $4\times 6$ array, called the MOG, Conway [Co1] found and analogous description of S(5,6,12) using a 3\times 4 array, called the MINIMOG. This section is devoted to the MINIMOG. The tetracode words are

0	0	0	0		0	+	+	+		0	-	-	-
+	0	+	-		+	+	-	0		+	-	0	+
-	0	-	+		-	+	0	-		-	-	+	0

With ”0″=0, “+”=1, “-“=2, these vectors form a linear code over GF(3). (This notation is Conway’s. One must remember here that “+”+”+”=”-“!) They may also be described as the set of all 4-tuples in of the form
where abc is any cyclic permutation of 012. The MINIMOG in the shuffle numbering is the array
\begin{array}{cccc} 6 & 3 & 0 & 9\\ 5 & 2 & 7 & 10 \\ 4 & 1 & 8 & 11 \end{array}
We label the rows of the MINIMOG array as follows:

  1. the first row has label 0,
  2. the second row has label +,
  3. the third row has label –

A col (or column) is a placement of three + signs in a column of the MINIMOG array. A tet (or tetrad) is a placement of 4 + signs having entries corresponding (as explained below) to a tetracode.

+	+	+	+
0	0	0	0
 	+	+	+
0	+	+	+
 	+	+	+

0	-	-	-

+	 	+	 

+	0	+	-
+	+	 	 

+	+	-	0
+	 	 	+

+	-	0	+
+	 	+	 

-	0	-	+

+	 	 	+

-	+	0	-

+	+	 	 

-	-	+	0

Each line in \mathbf{F}_3^2 with finite slope occurs once in the 3\times 3 part of some tet. The odd man out for a column is the label of the row corresponding to the non-zero digit in that column; if the column has no non-zero digit then the odd man out is a “?”. Thus the tetracode words associated in this way to these patterns are the odd men out for the tets. The signed hexads are the combinations $6$-sets obtained from the MINIMOG from patterns of the form

col-col, col+tet, tet-tet, col+col-tet.

Lemma 3 (Conway, [CS1], chapter 11, page 321) If we ignore signs, then from these signed hexads we get the 132 hexads of a Steiner system S(5,6,12). These are all possible $6$-sets in the shuffle labeling for which the odd men out form a part (in the sense that an odd man out “?” is ignored, or regarded as a “wild-card”) of a tetracode word and the column distribution is not 0,1,2,3 in any order.

Furthermore, it is known [Co1] that the Steiner system S(5,6,12) in the shuffle labeling has the following properties.

  1. There are 11 hexads with total 21 and none with lower total.
  2. The complement of any of these 11 hexads in \{0,1,...,11\} is another hexad.
  3. There are 11 hexads with total 45 and none with higher total.

Mathematical blackjack

Mathematical blackjack is a 2-person combinatorial game whose rules will be described below. What is remarkable about it is that a winning strategy, discovered by Conway and Ryba [CS2] and [KR], depends on knowing how to determine hexads in the Steiner system S(5,6,12) using the shuffle labeling.

Mathematical blackjack is played with 12 cards, labeled 0,\dots ,11 (for example: king, ace, 2, 3, …, 10, jack, where the king is 0 and the jack is 11). Divide the 12 cards into two piles of 6 (to be fair, this should be done randomly). Each of the 6 cards of one of these piles are to be placed face up on the table. The remaining cards are in a stack which is shared and visible to both players. If the sum of the cards face up on the table is less than 21 then no legal move is possible so you must shuffle the cards and deal a new game. (Conway [Co2] calls such a game *={0|0}, where 0={|}; in this game the first player automatically wins.)

  1. Players alternate moves.
  2. A move consists of exchanging a card on the table with a lower card from the other pile.
  3. The player whose move makes the sum of the cards on the table under 21 loses.

The winning strategy (given below) for this game is due to Conway and Ryba [CS2], [KR]. There is a Steiner system S(5,6,12) of hexads in the set \{0,1,...,11\}. This Steiner system is associated to the MINIMOG of in the “shuffle numbering” rather than the “modulo 11 labeling”.

The following result is due to Ryba.

Proposition 6: For this Steiner system, the winning strategy is to choose a move which is a hexad from this system.

This result is proven in a wonderful paper J. Kahane and A. Ryba, [KR]. If you are unfortunate enough to be the first player starting with a hexad from S(5,6,12) then, according to this strategy and properties of Steiner systems, there is no winning move! In a randomly dealt game there is a probability of 1/7 that the first player will be dealt such a hexad, hence a losing position. In other words, we have the following result.

Corollary 7: The probability that the first player has a win in mathematical blackjack (with a random initial deal) is 6/7.

An example game is given in this expository hexads_sage (pdf).


[Cu1] R. Curtis, “The Steiner system $S(5,6,12)$, the Mathieu group $M_{12}$, and the kitten,” in Computational group theory, ed. M. Atkinson, Academic Press, 1984.
[Cu2] —, “A new combinatorial approach to $M_{24}$,” Math Proc Camb Phil Soc 79(1976)25-42
[Co1] J. Conway, “Hexacode and tetracode – MINIMOG and MOG,” in Computational group theory, ed. M. Atkinson, Academic Press, 1984.
[Co2] —, On numbers and games (ONAG), Academic Press, 1976.
[CS1] J. Conway and N. Sloane, Sphere packings, Lattices and groups, 3rd ed., Springer-Verlag, 1999.
[CS2] —, “Lexicographic codes: error-correcting codes from game theory,” IEEE Trans. Infor. Theory32(1986)337-348.
[KR] J. Kahane and A. Ryba, “The hexad game,” Electronic Journal of Combinatorics, 8 (2001)