While GAP has a group_id function which locates a “small” group in a small groups database (see the SageMath page or the GAP page for more info), AFAIK, SageMath doesn’t have something similar. I’ve written one (see below) based on the mountain of hard work done years ago by Emily Kirkman.

def graph_id_graph6(Gamma, verbose=False):
"""
Returns the graph6 id of Gamma = (V,E).
If verbose then it also displays the table of all graphs with
|V| vertices and |E| edges.
Assumes Gamma is a "small" graph.
EXAMPLES:
sage: Gamma = graphs.HouseGraph()
sage: graph_id_graph6(Gamma, verbose=False)
'Dbk'
sage: graph_id_graph6(Gamma, verbose=True)
graphs with 5 vertices and 6 edges:
Graph6 Aut Grp Size Degree Sequence
------------------------------------------------------------
DB{ 2 [1, 2, 2, 3, 4]
DFw 12 [2, 2, 2, 3, 3]
DJ[ 24 [0, 3, 3, 3, 3]
DJk 2 [1, 2, 3, 3, 3]
DK{ 8 [2, 2, 2, 2, 4]
Dbk 2 [2, 2, 2, 3, 3]
'Dbk'
"""
n = len(Gamma.vertices())
m = len(Gamma.edges())
ds = Gamma.degree_sequence()
Q = GraphQuery(display_cols=['graph6', 'aut_grp_size', 'degree_sequence'], num_edges=['=', m], num_vertices=["=", n])
for g in Q:
if g.is_isomorphic(Gamma):
if verbose:
print("\n graphs with %s vertices"%n+" and %s edges:\n"%m)
Q.show()
print("\n")
return g.graph6_string()

The 1874 poem “The Mathematician in Love” by Scottish mechanical engineer William Rankine (in the book From Songs and Fables) has been published in many places (e.g., poetry.com, New Scientist and the scanned version is available at the internet archive. However, the mathematical equations Rankine presented at the end of his poem are only available in the scanned versions. As WordPress can render LaTeX, the poem quoted below includes those last few lines.

The Mathematician in Love
William J. M. Rankine

I.

A mathematician fell madly in love
With a lady, young, handsome, and charming:
By angles and ratios harmonic he strove
Her curves and proportions all faultless to prove,
As he scrawled hieroglyphics alarming.

II.

He measured with care, from the ends of a base.
The arcs which her features subtended:
Then he framed transcendental equations, to trace
The flowing outlines of her figure and face.
And thought the result very splendid.

III.

He studied (since music has charms for the fair)
The theory of fiddles and whistles, —
Then composed, by acoustic equations, an air,
Which, when ’twas performed, made the lady’s long hair
Stand on end, like a porcupine’s bristles.

IV.

The lady loved dancing: – he therefore applied.
To the polka and waltz, an equation;
But when to rotate on his axis he tried.
His centre of gravity swayed to one side.
And he fell, by the earth’s gravitation.

V.

No doubts of the fate of his suit made him pause.
For he proved, to his own satisfaction.
That the fair one returned his affection; – “because,
As every one knows, by mechanical laws,
Re-action is equal to action.”

VI.

“Let x denote beauty, – y manners well-bred, –
x, Fortune, – (this last is essential), –
Let L stand for love” – our philosopher said, –
“Then z is a function of x, y and 0,
Of the kind which is known as potential.”

VII.

“Now integrate L with respect to dt,
(t Standing for time and persuasion);
Then, between proper limits, ’tis easy to see,
The definite integral Marriage must be: —
(A very concise demonstration).”

VIII.

Said he – “If the wandering course of the moon
By Algebra can be predicted,
The female affections must yield to it soon” –
But the lady ran off with a dashing dragoon,
And left him amazed and afflicted.

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:

11 cards in the next row:

then 9 cards in the next row:

then 7 cards in the next row:

then 5 cards in the next row:

then 3 cards in the next row:

and finally, the remaining 4 cards in the last row:

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).

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.

Left to right: Oswald Veblen, Edmund Landau, Harald Bohr. Location, photographer, date unknown.

Now suppose we have a deck of (distictly labeled) cards, where is an integer. The collection of all possible shuffles, or permutations, of this deck is denoted 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 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 , write that element in disjoint cycle notation. Denote the lengths of the disjoint cycles occurring in as , where are integers forming a partition of : . Then the order of is known to be given by , where LCM denotes the least common multiple.

The Landau function is the function that returns the maximum possible order of an element . Landau introduced this function in a 1903 paper where he also proved the asymptotic relation .

Example: If then note and that .

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 180180

This was once posted on my USNA webpage. Since I’ve retired, I’m going to repost it here.

Coding Theory and Cryptography: From Enigma and Geheimschreiber to Quantum Theory (David Joyner, ed.) Springer-Verlag, 2000. ISBN 3-540-66336-3

Summary: These are the proceedings of the “Cryptoday” Conference on Coding Theory, Cryptography, and Number Theory held at the U. S. Naval Academy during October 25-26, 1998. This book concerns elementary and advanced aspects of coding theory and cryptography. The coding theory contributions deal mostly with algebraic coding theory. Some of these papers are expository, whereas others are the result of original research. The emphasis is on geometric Goppa codes, but there is also a paper on codes arising from combinatorial constructions. There are both, historical and mathematical papers on cryptography. Several of the contributions on cryptography describe the work done by the British and their allies during World War II to crack the German and Japanese ciphers. Some mathematical aspects of the Enigma rotor machine and more recent research on quantum cryptography are described. Moreover, there are two papers concerned with the RSA cryptosystem and related number-theoretic issues.

Chapters

Reminiscences and Reflections of a Codebreaker, by Peter Hilton pdf

This is now out of print and will not be reprinted (as far as I know). The above pdf files are posted by written permission. I thank Springer-Verlag for this.

In 2003, a math major named Steven McMath approached Fred Crabbe and I about directing his Trident thesis. (A Trident is like an honors thesis, but the student gets essentially the whole year to focus on writing the project.) After he graduated, I put a lot of his work online at the USNA website. Of course, I’ve retired since then and the materials were removed. This blog post is simply to try to repost a lot of the materials which arose as part of his thesis (which are, as official works of the US government, all in the public domain; of course, Dan Shanks notes are copyright of his estate and are posted for scholarly use only).

McMath planned to study Dan Shanks‘ SQUFOF method, which he felt could be exploited more than had been so far up to that time (in 2004). This idea had a little bit of a personal connection for me. Dan Shanks was at the University of Maryland during the time (1981-1983) I was a grad student there. While he and I didn’t talk as much as I wish we had, I remember him as friendly guy, looking a lot like the picture below, with his door always open, happy to discuss mathematics.

One of the first things McMath did was to type up the handwritten notes we got (I think) from Hugh Williams and Duncan Buell. A quote from a letter from Buell:

“Dan probably invented SQUFOF in 1975. He had just bought an HP 67 calculator, and the algorithm he invented had the advantages of being very simple to program, so it would fit on the calculator, very powerful as an algorithm when measured against the size of the program, and it factors double precision numbers using single precision arithmetic.“

Here are some papers on this topic (with thanks to Michael Hortmann for the references to Gower’s papers):

David Joyner, Notes on indefinite integral binary quadratic forms with SageMath, preprint 2004. pdf

F. Crabbe, D. Joyner, S. McMath, Continued fractions and Parallel SQUFOF, 2004. arxiv

D. Shanks, Analysis and Improvement of the Continued Fraction Method of Factorization, handwritten notes 1975, typed into latex by McMath 2004. pdf.

D. Shanks, An Attempt to Factor N = 1002742628021, handwritten notes 1975, typed into latex by McMath 2004. pdf.

D. Shanks, SQUFOF notes, handwritten notes 1975, typed into latex by McMath 2004. pdf.

S. McMath, Daniel Shanks’ Square Forms Factorization, 2004 preprint. pdf1, pdf2.

S. McMath, Parallel Integer Factorization Using Quadratic Forms, Trident project, 2004. pdf.

Since that time, many excellent treatments of SQUFOF have been presented. For example, see

J. Gower, S. Wagstaff, Square Form Factorization, Math Comp, 2008. pdf.

J. Gower, Square Form Factorization, PhD Thesis, December 2004. pdf.

At first, you might think this is obvious – just “clip” off each corner of the tetrahedron to create the truncated tetrahedron (by essentially creating a triangle from each of these clipped corners – see below for the associated graph). Then just map each such triangle to the corresponding vertex of the tetrahedron. No, it’s not obvious because the map just described is not a covering. This post describes one way to think about how to construct any covering.

First, color the vertices of the tetrahedron in some way.

The coloring below corresponds to a harmonic morphism :

All others are obtained from this by permuting the colors. They are all covers of – with no vertical multiplicities and all horizontal multiplicities equal to 1. These 24 harmonic morphisms of are all coverings and there are no other harmonic morphisms.

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 first, the 9 of Spades next (so it is below the 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

Spades

Diamonds

Clubs

6

9

Queen

2

5

8

Jack

Ace

4

7

10

King

3

6

9

Queen

2

5

8

Jack

Ace

4

7

10

King

3

6

9

Queen

2

5

8

Jack

Ace

4

7

10

King

3

6

9

Queen

2

5

8

Jack

Ace

4

7

10

King

3

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: .

For the ranks, identify King with 0, Ace with 1, 2 with 2, , 10 with 10, Jack with 11, Queen with 12. Therefore, the ranks may be identified with the additive group of integers (mod 13), namely: .

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

0

1

2

3

3

6

9

12

2

5

8

11

1

4

7

10

0

3

6

9

12

2

5

8

11

1

4

7

10

0

3

6

9

12

2

5

8

11

1

4

7

10

0

3

6

9

12

2

5

8

11

1

4

7

10

0

Mathematical version of the Si Stebbins Stack

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

.

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

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 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 , Hearts , Spades , Diamonds .

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 and the card drawn is then

For example, a player draws a card and you find that the bottom card is the . 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 (which is 3) to get (which is Clubs ). The card drawn is the .

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.

According to SageMath computations, there are 1544 connected, 4-regular graphs. Exactly 2 of these are symmetric (ie, arc transitive), also vertex-transitive and edge-transitive. Exactly 8 of these are vertex-transitive but not edge-transitive. None are distance regular.

Example 1: The first example of such a symmetric graph is the circulant graph with parameters (12, [1,5]), depicted below. It is bipartite, has girth 4, and its automorphism group has order 768, being generated by .

Example 2: The second example of such a symmetric graph is the cuboctahedral graph, depicted below. It has girth 3, chromatic number 3, and its automorphism group has order 48, being generated by .

In an earlier post titled Mathematical romantic? I mentioned some papers I inherited of one of my mathematical hero’s Andre Weil with his signature. In fact, I was fortunate enough to go to dinner with him once in Princeton in the mid-to-late 1980s – a very gentle, charming person with a deep love of mathematics. I remember he said he missed his wife, Eveline, who passed away in 1986. (They were married in 1937.)

All this is simply to motivate the question, why did I get these papers? First, as mentioned in the post, I was given Larry Goldstein‘s old office and he either was kind enough to gift me his old preprints or left them to be thrown away by the next inhabitant of his office. BTW, if you haven’t heard of him, Larry was a student of Shimura, when became a Gibbs Fellow at Yale, then went to the University of Maryland at COllege Park in 1969. He wrote lots of papers (and books) on number theory, eventually becoming a full professor, but eventually settled into computers and data science work. He left the University of Maryland about the time I arrived in the early 1980s to create some computer companies that he ran.

This motivates the question: How did Larry get these papers of Weil? I think Larry inherited them from Mountjoy (who died before Larry arrived at UMCP, but more on him later). This motivates the question, who is Mountjoy and how did he get them?

I’ve done some digging around the internet and here’s what I discovered.

The earliest mention I could find is when he was listed as a recipient of an NSF Fellowship in “Annual Report of the National Science Foundation: 1950-1953” under Chicago, Illinois, Mathematics, 1953. So he was a grad student at the University of Chicago in 1953. Andre Weil was there at the time. (He left sometime in 1958.) Mountjoy could have gotten the notes of Andre Weil then. Just before Weil left Chicago, Walter Lewis Baily arrived (in 1957, to be exact). This is important because in May 1965 the Notices of the AMS reported that reported:

Mountjoy, Robert Harbison Abelian varieties attached to representations of discontinuous groups (S. Mac Lane and W. L. Baily)

(His thesis was published posthumously in American Journal of Mathematics Vol. 89 (1967)149-224.) This thesis is in a field studied by Weil and Baily but not Saunders.

But we’re getting ahead of ourselves. The 1962 issue of Maryland Magazine had this:

Mathematics Grant A team of University of Maryland mathematics researchers have received a grant of $53,000 from the National Science Foundation to continue some technical investigations they started two years ago. The mathematical study they are directing is entitled “Problems in Geometric Function Theory.” The project is under the direction of Dr. James Hummel. Dr. Mischael Zedek. and Prof. Robert H. Mountjoy, all of the Mathematics Department. They are assisted by four graduate-student researchists. The $53,000 grant is a renewal of an original grant which was made two years ago.

We know he was working at UMCP in 1962.

Here’s the sad news.

The newspaper Democrat and Chronicle, from Rochester, New York, on Wednesday, May 25, 1965 (Page 40) published the news that Robert H. Mountjoy “Died suddenly at Purcellville, VA, May 23, 1965”. I couldn’t read the rest (it’s behind a paywall but I could see that much). The next day, they published more: “Robert H. Mountjoy, son-in-law of Mr and Mrs Allen P Mills of Brighton, was killed in a traffic crash in Virgina. Mountjoy, about 30, a mathematics instructors at the University of Maryland, leaves a widow Sarah Mills Mountjoy and a 5-month old son Alexander, and his parents Mr and Mrs Lucius Mountjoy of Chicago.”

It’s so sad. The saying goes “May his memory be a blessing.” I never met him, but from what I’ve learned of Mountjoy, his memory is indeed a blessing.

A little over 10 years ago, M. Baker and S. Norine (I’ve also seen this name spelled Norin) wrote a terrific paper on harmonic morphisms between simple, connected graphs (see “Harmonic morphisms and hyperelliptic graphs” – you can find a downloadable pdf on the internet of you google for it). Roughly speaking, a harmonic function on a graph is a function in the kernel of the graph Laplacian. A harmonic morphism between graphs is, roughly speaking, a map from one graph to another that preserves harmonic functions.

They proved quite a few interesting results but one of the most interesting, I think, is their graph-theoretic analog of the Riemann-Hurwitz formula. We define the genus of a simple connected graph to be

It represents the minimum number of edges that must be removed from the graph to make it into a tree (so, a tree has genus 0).

Riemann-Hurwitz formula (Baker and Norine): Let be a harmonic morphism from a graph to a graph . Then

I’m not going to define them here but denotes the horizontal multiplicity and denotes the vertical multiplicity.

I simply want to record a very easy corollary to this, assuming is -regular and is -regular.

Corollary: Let be a non-trivial harmonic morphism from a connected -regular graph to a connected -regular graph. Then

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