Number of groups of given order

Definition

Let $n$ be a natural number. The number of groups of order $n$ is defined as the number of isomorphism classes of groups whose order is $n$.

This is a finite number and is bounded by $n^{2n}$ for obvious reasons. The function is not strictly increasing in $n$ and depends heavily on the nature of the prime factorization of $n$.

Initial values

The ID of the sequence of these numbers in the Online Encyclopedia of Integer Sequences is A000001

$n$	Number of groups of order $n$	Reason/explanation
1	1
2	1	prime number
3	1	prime number
4	2	square of a prime; see classification of groups of prime-square order
5	1	prime number
6	2	form $pq$ where $p,q$ primes, $q\mid p-1$
7	1	prime number
8	5	prime cube: classification of groups of prime-cube order, also see groups of order 8
9	2	prime square; see [[classification of groups of prime-square order

[SHOW MORE]

We omit the prime numbers since there is only one group of each such order.

$n$	Number of groups of order $n$	Reason/explanation
10	2	form $pq$ where $p,q$ primes, $q\mid p-1$
12	5	see groups of order 12
14	2	form $pq$ where $p,q$ primes, $q\mid p-1$
15	1	form $pq$ ($p,q$ primes) where $p$ doesn't divide $q-1$, $q$ doesn't divide $p-1$
16	14	see groups of order 16
18	5	see groups of order 18
20	5	see groups of order 20
21	2	form $pq$ where $p,q$ primes, $q\mid p-1$
22	2	form $pq$ where $p,q$ primes, $q\mid p-1$
24	15	see groups of order 24
25	2	prime square; see classification of groups of prime-square order
26	2	form $pq$ where $p,q$ primes, $q\mid p-1$
27	5	see classification of groups of prime-cube order
28	4
30	4
32	51
33	1	form $pq$ ($p,q$ primes) where $p$ doesn't divide $q-1$, $q$ doesn't divide $p-1$
34	2	form $pq$ where $p,q$ primes, $q\mid p-1$
35	1	form $pq$ ($p,q$ primes) where $p$ doesn't divide $q-1$, $q$ doesn't divide $p-1$
36	14	see groups of order 36

Facts

Basic facts

Value of $n$	What we can say about the number of groups of order $n$	Explanation
1	1	only the trivial group
$p$ a prime number	1	only the group of prime order. See equivalence of definitions of group of prime order
$p^{2}$, $p$ prime	2	only the cyclic group of prime-square order and the elementary abelian group of prime-square order
$p^{3}$, $p$ prime	5	see classification of groups of prime-cube order
$2^{4}=16$	14	see classification of groups of order 16, also groups of order 16 for summary information.
$p^{4}$, $p$ odd prime	15	see classification of groups of prime-fourth order for odd prime
product $p_{1}p_{2}\dots p_{n}$, $p_{i}$ distinct primes with no $p_{i}$ dividing $p_{j}-1$	1	the cyclic group of that order. See classification of cyclicity-forcing numbers

Asymptotic fact

• Higman-Sims asymptotic formula on number of groups of prime power order
• Pyber's theorem on logarithmic quotient of number of nilpotent groups to number of groups approaching unity

Properties

Supermultiplicativity

If $n=ab$ with $a$ and $b$ relatively prime, the number of groups of order $n$ is bounded from below by the product of the number of groups of orders $a$ and $b$ respectively. This is because we can take direct products for every pair of a group of order $a$ and a group of order $b$.