A209401 Number of noncommutative rings with n elements.

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

Offset: 1

Author

Ben Branman, Mar 26 2012

Keywords

• nonn
• nice
• hard
• more

Comments

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

Examples

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

Links

• Gregory Dresden, Rings of Size Four, demonstrating that a(4)=2.

Crossrefs

Cf. A027623, A037289, A037291.

Formula

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