

A209401


Number of noncommutative rings with n elements.


0



0, 0, 0, 2, 0, 0, 0, 18, 2, 0, 0, 4, 0, 0, 0, 228, 0, 4, 0, 4, 0, 0, 0, 36, 2, 0, 23, 4, 0, 0, 0
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OFFSET

1,4


COMMENTS

a(n)=0 if and only if n is squarefree.


LINKS

Table of n, a(n) for n=1..31.
Gregory Dresden, Rings of Size Four, demonstrating that a(4)=2.


FORMULA

a(n) = A027623(n)  A037289(n).


EXAMPLE

For n=8, there are 52 rings of order 8, 18 of which are noncommutative, so a(8)=18.


CROSSREFS

Cf. A027623, A037289, A037291.
Sequence in context: A136615 A283950 A277443 * A029696 A118887 A057383
Adjacent sequences: A209398 A209399 A209400 * A209402 A209403 A209404


KEYWORD

nonn,nice,hard,more


AUTHOR

Ben Branman, Mar 26 2012


STATUS

approved



