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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified April 8 09:44 EDT 2020. Contains 333313 sequences. (Running on oeis4.)