# Groups of order 2019

How many groups have 2019 elements? What are these groups?

2019 is a semiprime, i.e. the product of two primes, 3 and 673. For every semiprime s, there are either one or two distinct groups of order s.

As explained here, if spq with pq, all groups of order s are isomorphic if q is not a factor of p-1. If q does divide p-1 then there are exactly two non-isomorphic groups of order s, one abelian and one non-abelian. In our case, 3 does divide 672, so there are two groups of order 2019. The first is easy: the cyclic group of order 2019. The second is more complex.

You could take the direct product of the cyclic groups of order 3 and 673, but that turns out to be isomorphic to the cyclic group of order 2019. But if you take the semidirect product you get the other group of order 2019, the non-abelian one.

## Semidirect products

Starting with two groups G and H, the direct product G × H is the Cartesian product of G and H with multiplication defined componentwise. The semidirect product of G and H, written [1]

also starts with the Cartesian product of G and H but defines multiplication differently.

In our application, G will be the integers mod 673 with addition and H will be a three-element subgroup of the integers mod 673 with multiplication [2]. Let H be the set {1, 255, 417} with respect to multiplication [3]. Note that 1 is its own inverse and 255 and 417 are inverses of each other.

### Product

Now the group product of (g1, h1) and (g2, h2) is defined to be

(g1 + h1-1g2, h1 h2)

So, for example, the product of (5, 255) and (334, 417) is (5 + 417*334, 255*417) which reduces to (645, 1) working mod 673.

(We haven’t defined the semidirect product in general, but the procedure above suffices to define the semidirect product for any two groups of prime order, and so it is sufficient to find all groups of semiprime order.)

Note that our group is non-abelian. For example, if we reverse the order of multiplication above we get (263, 1).

### Identity

The identity element is just the pair consisting of the identity elements from G and H. In our case, this is (0, 1) because 0 is the additive identity and 1 is the multiplicative identity.

### Inverse

The inverse of an element (gh) is given by

(-ghh-1).

So, for example, the inverse of (600, 255) is (444, 417).

## Python code

The goal of this post is to be concrete rather than general.

So to make everything perfectly explicit, we’ll write a little Python code implementing the product and inverse.


def hinv(h):
if h == 255:
return 417
if h == 417:
return 255
if h == 1:
return 1

def prod(x, y):
g1, h1 = x
g2, h2 = y
g3 = (g1 + hinv(h1)*g2) % 673
h3 = (h1*h2) % 673
return (g3, h3)

def group_inv(x):
g, h = x
return ((-g*h)%673, hinv(h))

x = (5, 255)
y = (334, 417)
print(prod(x, y))
print(prod(y, x))
print(group_inv((600, 255)))


The following code verifies that our group satisfies the axioms of a group.

    from itertools import product

identity = (0, 1)
h_list = [1, 255, 417]

def elem(x):
g, h = x
g_ok = (0 <= g <= 672)
h_ok = (h in h_list)
return (g_ok and h_ok)

group = product(range(673), h_list)
assert(len([g for g in group]) == 2019)

# closed under multiplicaton
for x in group:
for y in group:
assert( elem(prod(x, y)) )

# multiplication is associative
for x in group:
for y in group:
for z in group:
xy_z = prod(prod(x, y),z)
x_yz = prod(x, prod(y,z))
assert(xy_z == x_yz)

# identity acts like it's supposed to
for x in group:
assert( prod(x, identity) == x )
assert( prod(identity, x) == x )

# every element has an inverse
for x in group:
ginv = group_inv(x)
assert( elem(ginv) )
assert( prod(x, ginv) == identity )
assert( prod(ginv, x) == identity )


## Related posts

[1] The symbol for semidirect product is ⋊. It’s U+22CA in Unicode and \rtimes in LaTeX.

[2] In general, the semidirect product depends on a choice of an action of the group H on the group G. Here the action is multiplication by an element of H. Different actions can result in different groups. Sometimes the particular choice of action is made explicit as a subscript on the ⋊ symbol.

[3] How did I find these numbers? There are 672 non-zero numbers mod 673, so I picked a number, it happened to be 5, and raised it to the powers 672/3 and 2*672/3.

# Groups of semiprime order

For each prime p, there is only one group with p elements, the cyclic group with that many elements. It would be plausible to think there is only one group of order n if and only if n is prime, but this isn’t the case.

If p and q are primes, then there are ostensibly at least two groups of order pq: the cyclic group Zpq, and Zp + Zq, the direct sum of the cyclic groups of orders p and q. However, there may just be one group of order pq after all because the two groups above could be isomorphic.

If pq = 2, then Z4 and Z2 + Z2 are not isomorphic. But the groups Z15 and Z3 + Z5 are isomorphic. That is, there is only one group of order 15, even though 15 is composite. This is the smallest such example.

Let p and q be primes with pq. If q does not divide p-1, then there is only one group of order pq. That is, all groups of order pq are isomorphic to the cyclic group Zpq. So when p = 5 and q = 3, there is only one group of order 15 because 3 does not evenly divide 5-1 = 4. The same reasoning shows, for example, that there must only be one group with 77 elements because 7 does not divide 10.

Now if q does divide p-1, then there are two distinct groups of order pq. One is the cyclic group with pq elements. But the other is non-Abelian, and so it cannot be Zp + Zq. So once again Zpq is isomorphic to Zp + Zq, but there’s a new possibility, a non-Abelian group.

Note that this does not contradict our earlier statement that Z4 and Z2 + Z2 are different groups, because we assumed p > q. If pq, then Zpq is not isomorphic to Zp + Zq.

# Number of groups of prime power order

John Baez left a comment on my post on group statistics saying

It’s known that the number of groups of order p^n for prime p is p^{2n^3/27+O(n^(8/3))}. It might be fun to compare this to what Mathematica says.

Here goes. First let’s let p = 2. Mathematica’s FiniteGroupCount function tops out at n = 10. And for the range of values Mathematica does support, 2^{2n^3/27} grossly overestimates the number of groups.

    Table[{FiniteGroupCount[2^n], 2^((2./27) n^3)}, {n, 1, 11}]

{{1, 1.05269}, {2, 1.50795}, {5, 4.}, {14, 26.7365},
{51, 612.794}, {267, 65536.}, {2328, 4.45033*10^7},
{56092, 2.61121*10^11}, {10494213, 1.80144*10^16},
{49487365422, 1.98847*10^22}, {FiniteGroupCount[2048], 4.7789*10^29}}


OEIS has entries for the sizes of various groups, and the entry for powers of 2 confirms the asymptotic formula above. Maybe it’s not a good approximation until n is very large. (Update: Here’s a reference for the asymptotic expression for all p.)

The OEIS entry for number of groups of order powers of 5 has an interesting result:

For a prime p >= 5, the number of groups of order p^n begins 1, 1, 2, 5, 15,
61 + 2*p + 2*gcd (p – 1, 3) + gcd (p – 1, 4),
3*p^2 + 39*p + 344 + 24*gcd(p – 1, 3) + 11*gcd(p – 1, 4) + 2*gcd(p – 1, 5), …

We can duplicate this with Mathematica.

    Table[FiniteGroupCount[5^n], {n, 0, 6}]

{1, 1, 2, 5, 15, 77, 684}

and the last two numbers match the calculations given in OEIS.

There’s something interesting going on with Mathematica. It doesn’t seem to know, or agree with, the formula above for groups of order p4. For example,

    Table[FiniteGroupCount[7^n], {n, 0, 6}]

{1, 1, 2, 5, FiniteGroupCount[2401], 83, 860}


I get similar results when I use larger primes: it can’t handle the fourth power.

    Table[FiniteGroupCount[389^n], {n, 0, 6}]

{1, 1, 2, 5, FiniteGroupCount[22898045041], 845, 469548}


The results for n = 5 and 6 agree with OEIS.

Is OEIS wrong about the number of groups of order  p4 or should Mathematica simply return 15 but there’s a gap in the software?

Also, does anybody know why the agreement with the asymptotic formula above is so bad? It’s just as bad or worse for other primes that I tried.

# Group statistics

I just ran across FiniteGroupData and related functions in Mathematica. That would have made some of my earlier posts easier to write had I used this instead of writing my own code.

Here’s something I find interesting. For each n, look at the groups of order at most n and count how many are Abelian versus non-Abelian. At first there are more Abelian groups, but the non-Abelian groups soon become more numerous. Also, the number of Abelian groups grows smoothly, while the number of non-Abelian groups has big jumps, particularly at powers of 2.

Here’s the Mathematica code:

    fgc = FoldList[Plus, 0, Table[FiniteGroupCount[n], {n, 1, 300}]]
fga = FoldList[Plus, 0, Table[FiniteAbelianGroupCount[n], {n, 1, 300}]]
ListLogPlot[ {fgc - fga, fga },
PlotLegends -> {"Non-Abelian", "Abelian"},
Joined -> True,
AxesLabel -> {"order", "count"}]


I see the plot legend on my screen, but when saving the plot to a file the legend wasn’t included. Don’t know why. (Update: See footnote [1]). The jagged blue curve is the number of non-Abelian groups of size up to n. The smooth gold curve is the corresponding curve for Abelian groups.

Here’s the same plot carried out further to show the jumps at 512 and 1024.

## Related posts

[1] Someone from Wolfram Research saw this post and sent me a fix:

pl = ListLogPlot[...]
Export["~/Desktop/img.png", pl]


# Overlap in the classification of finite simple groups

The previous post defined the groups PSL(nq) where n is a positive integer and q is a prime power. These are finite simple groups for n ≥ 2 except for PSL(2, 2) and PSL(2, 3).

## Overlap among PSL(n, q)

There are a couple instances where different values of n and q lead to isomorphic groups: PSL(2, 4) and PSL(2, 5) are isomorphic, and PSL(2, 7) and PSL(3, 2) are isomorphic. These are the only instances [1].

With the exceptions stated above, distinct values of n and q lead to distinct groups. Is it possible for different choices of n and q to lead to groups of the same size, even though the groups are not isomorphic to each other? Yes, PSL(3, 4) and PSL(4, 2) both have order 20160, but the groups are not isomorphic. This is the only example [2].

## Overlap between PSL and alternating groups

The first post in this series mentioned that for n ≥ 5, the alternating group An, the group of even permutations on a set of n elements, is a simple group. Three of the alternating groups are isomorphic to PSL groups:

• PSL(2, 4) = PSL(2, 5) = A5
• PSL(2, 9) = A6
• PSL(4, 2) = A8

Here “=” really means isomorphic. We mentioned PSL(4, 2) above. It has the same order as PSL(3, 4). This means that A8 and PSL(3, 4) have the same order but are not isomorphic.

I suspect that with a small number of exceptions, the order of a finite simple group determines the group. I haven’t proven that, but numerical exploration suggests its true. This page lists non-Abelian finite simple groups of order less than 10 billion, and there are only seven orders that correspond to more than 1 group, the largest example being order 25,920.

## One last overlap

There is only one other duplication in the lists of groups in the CFSG theorem, and that is PSU(4, 2) = PSp(4, 3). I haven’t written about these groups yet.

## Notes

[1] See The Finite Simple Groups by Robert A. Wilson

[2] In fact, aside from the groups mentioned in this post, the orders of all the finite simple groups are unique except for two non-isomorphic families that have orders: PΩ2n+1(q) and PSp2n(q) for n ≥ 3 and odd prime powers q. See discussion on Math Overflow.

# Orders of finite simple groups

Simple groups are to groups as prime numbers are to numbers.A simple group has no non-trivial normal subgroups, just as a prime number has no non-trivial factors.

# A tale of two elliptic curves

A few days ago I blogged about the elliptic curve secp256k1 and its use in Bitcoin. This curve has a sibling, secp256r1. Note the “r” in the penultimate position rather than a “k”. Both are defined in SEC 2: Recommended Elliptic Curve Domain Parameters. Both are elliptic curves over a field zp where p is a 256-bit prime (though different primes for each curve).

The “k” in sepc256k1 stands for Koblitz and the “r” in sepc256r1 stands for random. A Koblitz elliptic curve has some special properties that make it possible to implement the group operation more efficiently. It is believed that there is a small security trade-off, that more “randomly” selected parameters are more secure. However, some people suspect that the random coefficients may have been selected to provide a back door.

Both elliptic curves are of the form y² = x³ + ax + b. In the Koblitz curve, we have

a = 0
b = 7


and in the random case we have

a = FFFFFFFF 00000001 00000000 00000000 00000000 FFFFFFFF FFFFFFFF FFFFFFFC
b = 5AC635D8 AA3A93E7 B3EBBD55 769886BC 651D06B0 CC53B0F6 3BCE3C3E 27D2604B


You can find the rest of the elliptic curve parameters in the SEC 2 report. For some help understanding what the parameters mean and how to decode them, see my earlier post.

The NSA recommends the random curve for government use. It is also known as P-256. Or rather it did recommend P-256 as part of its Suite B of cryptography recommendations. In August 21015 the NSA announced its concern that in the future, quantum computing could render the Suite B methods insecure. As far as we know, quantum computing at scale is years, maybe decades, away. But it takes a long time to develop quality encryption methods, and so the NSA and NIST are urging people to think ahead.

Bitcoin chose to use the less popular Koblitz curve for the reasons mentioned above, namely efficiency and concerns over a possible back door in the random curve. Before Bitcoin, secp256k1 was not widely used.

Related post: RSA numbers and factoring

# Schnorr groups

Schnorr groups, named after Claus Schnorr, are multiplicative subgroups of a finite field. They are designed for cryptographic application, namely as a setting for hard discrete logarithm problems.

The application of Schnorr groups to Schnorr signatures was patented, but the patent ran out in 2008.

There has been a proposal to include Schnorr signatures in Bitcoin, but it has not been accepted at this point. (The proposal would use Schnorr signatures but over an elliptic curve rather than a Schnorr group.)

## Group construction

Pick a large prime p. Then the integers mod p form a finite field. The non-zero elements form an Abelian group of order p-1 under multiplication. Then p – 1 is composite, and so it can be factored into qr where q is prime. We want q to be large as well because it will be the order of our Schnorr group.

Pick any h such that hr is not congruent to 1 mod p. Then g = hr is the generator of our Schnorr group. Why does g have order q? A simple calculation shows

where the last step follows from Fermat’s little theorem. This shows that the order of g is either q or a divisor of q, but by construction g is not congruent to 1 (mod p), and q has no other factors since it is prime.

## Python code

Here’s a toy example using small numbers. Let p = 23, q = 11 and r = 2. Then we can pick h to be any number that is not a root of 1 (mod 23), i.e. any number except 1 or 22. Say we pick h = 7. Then g = 49 = 3 (mod 23).

Here’s a Python script to verify our claims and print out the elements of our Schnorr group.

    from sympy import isprime

p, q, r, h = 23, 11, 2, 7

assert(isprime(p))
assert(isprime(q))
assert(p-1 == q*r)

g = h**r % p
assert(g != 1)
assert(g**q % p == 1)

for i in range(q):
print( g**i % p )


This shows that our group elements are {1, 3, 9, 4, 12, 13, 16, 2, 6, 18, 8}.

In theory we could use the same script with much larger numbers, but in that case we wouldn’t want to print out the elements. Also, the code would take forever. We naively computed ab (mod m) by computing ab first, then taking the remainder modulo m. It is far more efficient to reduce modulo m along the way. That’s what Python’s three-argument pow function does.

Here’s a revised version that runs quickly with large numbers.

    from sympy import isprime

p = 115792089237316195423570985008687907852837564279074904382605163141518161494337
q = 341948486974166000522343609283189
r = 338624364920977752681389262317185522840540224
h = 3141592653589793238462643383279502884197

assert(isprime(p))
assert(isprime(q))
assert(p-1 == q*r)

g = pow(h, r, p)
assert(g != 1)
assert(pow(g, q, p) == 1)


## Related posts

Photo of Claus Schnorr by Konrad Jacobs CC BY-SA 2.0 de

# Summarizing homotopy groups of spheres

I don’t understand homotopy groups of spheres, and it’s OK if you don’t either. Nobody fully understands them. This post is really more about information compression than homotopy. That is, I’ll be looking at ways to summarize what is known without being overly concerned about what the results mean.

## The task: map two integers to a list of integers

For each positive integer k, and non-negative integer n, the kth homotopy group of the sphere Sn is a finitely generated Abelian group, something that can be described by a finite list of numbers. So we’re looking at simply writing a function that takes two integers as input and returns a list of integers. This function is implemented in an online calculator that lets you lookup homotopy groups.

Computing homotopy groups of spheres is far from easy. The first Fields medal given to a topologist was for partial work along these lines. There are still groups that haven’t been computed, and potentially more Fields medals to win. But our task is much more modest: simply to summarize what has been discovered.

This is not going to be too easy, as suggested by the sample of results in the table below.

This table was taken from the Homotopy Type Theory book, and was in turn based on the Wikipedia article on homotopy groups of spheres.

## Output data representation

To give an example of what we’re after, the table says that π13(S²), the 13th homotopy group of the 2-sphere, is Z12 × Z2. All we need to know is the subscripts on the Z‘s, the orders of the cyclic factors, and so our function would take as input (13, 2) and return (12, 2).

The table tells us that π8(S4) = Z22. This is another way of writing Z2 × Z2, and so our function would take (8, 4) as input and return (2, 2).

When I said above that our function would return a list of integers I glossed over one thing: some of the Z‘s don’t have a subscript. That is some of the factors are the group of integers, not the group of integers mod some finite number. So we need to add an extra symbol to indicate a factor with no subscript. I’ll use ∞ because the integers as the infinite cyclic group. For example, our function would take (7, 4) and return (∞, 12). I will also use 1 to denote the trivial group, the group with 1 element.

Some results are unknown, and so we’ll return an empty list for these.

The order of the numbers in the output doesn’t matter, but we will list the numbers in descending order because that appears to be conventional.

## Theorems

Some of the values of our function can be filled in by a general theorem, and some will simply be data.

If we call our function f, then there is a theorem that says f(kn) = (1) if kn.  This accounts for the zeros in the upper right corner of the chart above.

There’s another theorem that says f(n+mn) is independent of n if n > m + 1. These are the so-called stable homotopy groups.

The rest of the groups are erratic; we can’t do much better than just listing them as data.

(By the way, the corresponding results for homology rather than homotopy are ridiculously simple by comparison. For k > 0, the kth homology group of Sn is isomorphic to the integers if k = n and is trivial otherwise.)

# Weakening the requirements of a group

A group is a set with a binary operation that

1. closed
2. associative,
3. has an identity, and
4. has inverses.

This post will look at each of these properties and what happens if you modify or remove it.

## Magmas

Closed means that applying the binary operation to any two elements of the set yields another elements of the set. (I saw a book one time that had at the top of some chapter a quote something like “The product of two real numbers is a real number; we should be surprised if it were a head of cabbage.”) For example, the positive integers are closed under multiplication but not under division.

A set with a closed binary operation is called a magma.

## Semigroups

Associative means that you can parenthesize an expression any way you want and always get the same result. It may be practically important how you parenthesize an expression. For example, matrix multiplication can far fewer operations if you carry out the multiplications in the right order. In programming, left folds and right folds (such as foldl and foldr in Haskell) may have very different efficiency, but they’re supposed to return the same result if you’re folding an associative operation.

A magma with an associative operation is called a semigroup.

Differential equation theory is concerned with analytic semigroups. If you advance the solution of an evolution equation by a positive time increment t and then by an increment s, you end up at the same place as if you’d advanced the solution by ts.

Notice that the time increment is required to be positive. That’s because the initial conditions for an evolution equation might reside in a different function space than the solutions at any future time. That’s the case, for example, with the heat equation, where initial conditions can be rough but the solution smooths out immediately as you move forward in time.

## Monoids

If you add the requirement that a semigroup has an identity, you get a monoid. Partial differential equation theory is more concerned with analytic semigroups than analytic monoids for the reason explored above: you might not have an identity.

Positive integers with multiplication form a monoid but not a group because they have an identity, 1, but positive integers other than 1 do not have a multiplicative inverse that is an integer.

A monoid in which every element has an inverse is a group.

## Quasigroups

So far we’ve built up a progression of stronger and stronger requirements leading up to groups, starting with requiring that our operation is associative. What if we don’t have associativity, but we do have other properties?

Non-associative operations are less common. Perhaps the best known operation that is not associative is octonion multiplication.

There are four division algebras over the reals: the reals themselves, complex numbers, quaternions, and octonions. Real and complex numbers are both fields. Quaternions are not a field because multiplication is not commutative. Octonions are even further from being a field because multiplication is not even associative.

But division is possible for octonions, except for 0, and so non-zero octonions form a quasigroup, a magma with division.

## Loops

A quasigroup that has an identity element is called a loop. Note that for quasigroups we said “division is possible,” not that elements have inverses. If elements do have inverses, then you can divide by an element by multiplying by its inverse. But you can define division by the ability to solve equations. We say you can divide b by a if axb and yab have unique solutions.

A loop with associativity is a group.

## Alternative loops

Loops are not required to have an associative property, but they may satisfy a weak form of associativity.

An alternative loop satisfies x(xy) = (xx)y and x(yy) = (xy)y. Non-zero octonions form an alternative loop with respect to multiplication.

## Moufang loops

A Moufang loop is a loop that satisfies these requirements:

1. z(x(zy)) = ((zx)z)y
2. x(z(yz)) = ((xz)y)z
3. (zx)(yz) = (z(xy))z
4. (zx)(yz) = z((xy)z)

These conditions imply the alternative loop conditions, so Moufang loops are alternative loops.

Moufang loop are still not required to be associative, but they’re getting closer. And as you might have anticipated, non-zero octonions with respect to multiplication form a Moufang loop.

# Defining the Fourier transform on LCA groups

My previous post said that all the familiar variations on Fourier transforms—Fourier series analysis and synthesis, Fourier transforms on the real line, discrete Fourier transforms, etc.—can be unified into a single theory. They’re all instances of a Fourier transform on a locally compact Abelian (LCA) group. The difference between them is the underlying group.

Given an LCA group G, the Fourier transform takes a function on G and returns a function on the dual group of G. We said this much last time, but we didn’t define the dual group; we just stated examples. We also didn’t say just how you define a Fourier transform in this general setting.

## Characters and dual groups

Before we can define a dual group, we have to define group homomorphisms. A homomorphism between two groups G and H is a function h between the groups that preserves the group structure. Suppose the group operation is denoted by addition on G and by multiplication on H (as it will be in our application), saying h preserves the group structure means

h(x + y) = h(x) h(y)

for all x and y in G.

Next, let T be the unit circle, i.e. complex numbers with absolute value 1. T is a group with respect to multiplication. (Why T for circle? This is a common notation, anticipating generalization to toruses in all dimensions. A circle is a one-dimensional torus.)

Now a character on G is a continuous homomorphism from G to T. The set of all characters on G is the dual group of G. Call this group Γ. If G is an LCA group, then so is Γ.

## Integration

The classical Fourier transform is defined by an integral. To define the Fourier transform on a group we have to have a way to do integration on that group. And there’s a theorem that says we can always do that. For every LCA group, there exists a Haar measure μ, and this measure is nice enough to develop our theory. This measure is essentially unique: Any two Haar measures on the same LCA group must be proportional to each other. In other words, the measure is unique up to multiplying by a constant.

On a discrete group—for our purposes, think of the integers and the integers mod m—Haar measure is just counting; the measure of a set is the number of things in the set. And integration with respect to this measure is summation.

## Fourier transform defined

Let f be a function in L¹(G), i.e. an absolutely integrable function on G. Then the Fourier transform of f is a function on Γ defined by

What does this have to do with the classical Fourier transform? The classical Fourier transform takes a function of time and returns a function of frequency. The correspondence between the classical Fourier transform and the abstract Fourier transform is to associate the frequency ω with the character that takes x to the value exp(iωx).

There are multiple slightly different conventions for the classical Fourier transform cataloged here. These correspond to different constant multiples in the choice of measure on G and Γ, i.e. whether to divide by or multiply by √(2π), and in the correspondence between frequencies and characters, whether ω corresponds to exp(±iωx) or exp(±2πiωx).

# Safe primes, Sylow theorems, and Cryptography

A logarithm is the inverse of an exponential, and so we can generalize the idea of a logarithm wherever we can generalize the idea of an exponential. In particular, we can raise elements to exponents in a discrete group, and that leads to the definition of a discrete logarithm.

Diffie-Hellman public key cryptography is based on the assumption that discrete logarithms are hard to compute. There are algorithms to compute discrete logarithms that are much faster than brute force. For example, baby-step giant-step is a fairly simple algorithm. There are more efficient algorithms as well, but the best known algorithms are still much slower than raising numbers to powers. Whenever you find something that is much harder to undo than to do, it might be useful in cryptography, and that is the case with discrete logs.

Diffie-Hellman encryption requires users to compute exponentials and presumably requires attackers to compute discrete logs. I say “presumably” because it’s a fatal error in cryptography to assume an attacker has to solve the problem you think he’d have to solve. For example, you can create a simple encryption scheme by permuting the alphabet and encrypting each letter to its counterpart in the permutation. Someone might naively think “No one can break this because they’d have to try 26! permutations of the alphabet, over 400 million million million million possibilities!” Except that’s not how anyone approaches a substitution cipher. If it were, you wouldn’t see cryptograms in puzzle books.

As far as we know, discrete logarithms are hard to compute when working over integers mod p where p is a large prime, except for primes that have certain properties. We’ll look at what those properties are below and how to avoid them.

For a prime p, the integers mod p form a finite field. They are a group under addition, and the non-zero elements form a group under multiplication. It’s the multiplicative group we care about here. This group has order p-1, i.e. it has p-1 elements.

A group of prime order has no proper subgroups. But a group of composite order does. And our multiplicative group has order p-1, which is composite. (Except for p = 3, and cryptography depends on primes far, far bigger than 3.)

Sylow’s theorems tell us something about what kinds of subgroups a group must have. If s is prime and sk is a factor of the order of our group, then the group has a subgroup of order sk. We don’t want our multiplicative group to have any small-order subgroups because these would make it easier to compute discrete logarithms.

A safe prime p has the form 2q + 1 where q is also prime [1]. Diffie-Hellman chooses safe primes for moduli because this means the multiplicative group of order p-1 = 2q has no small subgroups. (It has two small subgroups, {1} and {1, -1}, but these can easily be avoided. The algorithm requires picking a generator g, and as long as you don’t pick g to be 1 or -1 mod p, then g generates a group of order q, and if p is gigantic, so is q.) Because q is prime, the subgroup of order q does not have any further subgroups.

Related post: Probability that a number is prime

[1]  If q and p = 2q + 1 are both prime, q is called a Sophie Germain prime and p is a safe prime.

# Visualizing Galois groups of quadratics

Yesterday Jack Kennedy told me about a graph he’d made as part of a project he’s working on and I asked if I could post it here.

The Galois group of a quadratic polynomial x2 + bx + c is either A2 or S2. If b2 – 4c is a perfect square, the polynomial has rational roots and the Galois group is the trivial group A2. Otherwise there are distinct irrational roots and the Galois group is the two-element group S2.

As b and c range over integers, color a pixel yellow if the group is A2 and black otherwise. This produces the image below.

Note that what appear to be the crossed lines y = ±x intersecting at 0 are actually the lines y = ±(x+1) intersecting at (-1,0).

You can find a larger image here. View the page source to see the JavaScript that produced the image. The page is calculating and setting the value of one million pixels, and yet the time to render the page isn’t even noticeable.

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# Normal subgroups are not transitive

The property “is a normal subgroup of” is not transitive.

If A is a subgroup of B, and B is a subgroup of C, then A is a subgroup of C. But the corresponding statement about normal subgroups is false. And there’s a simple example that shows it is false.

We need to find a group C with subgroups A and B such that A is normal in B, B is normal in C, but A is not normal in C.

The subgroup A must have at least two elements, otherwise A would just be the group identity and would then be a normal subgroup of C. The order of a subgroup divides the order of the group, so B must have at least twice as many elements as A, and C must have twice as many elements as B. So the smallest possible example would be a group with 8 elements and subgroups of order 2 and 4.

We’re in luck, because there’s a group of order 8 that will work, D8. This is the group of symmetries of a square under flips and rotations. Let A be the subgroup of flips about the vertical axis of symmetry. Let B the symmetries you can find by combinations of such flips and 180 degree rotations. You can show that A is normal in B, and B is normal in C.

Now let c be a 90 degree clockwise turn and let a be a flip. You can show that cac-1 is not a flip or the identity, so A is not a normal subgroup of C.

Related post: A 3,000 page proof (classification of finite simple groups)

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