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 A127708 Number of non-commutative rings with 1 containing n elements. 11
 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 13, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 99, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 13, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,16 COMMENTS We consider rings in which multiplication is associative and has a unit, but where there is at least one pair of non-commuting elements. LINKS C. Noebauer, Home page [Archived copy as of 2008 from web.archive.org] FORMULA a(n) = A037291(n) - A127707(n). - Bernard Schott, Apr 19 2022 EXAMPLE a(n)=0 for n<=7 and a(8)=1, so all rings (with unit) of cardinality at most 7 are commutative, while the smallest non-commutative ring (with unit) has cardinality 8 and is unique up to isomorphism; it can be represented as the ring of upper-triangular matrices of size 2 over F_2. A037291(32) = 208, A127707(32) = 109, hence a(32) = 208 - 109 = 99. CROSSREFS Cf. A027623, A037291, A127707, A209401, A342376. Sequence in context: A165970 A285180 A081357 * A094896 A277118 A200065 Adjacent sequences: A127705 A127706 A127707 * A127709 A127710 A127711 KEYWORD nonn,nice,more AUTHOR Hugues Randriam (randriam(AT)enst.fr), Jan 24 2007, Jan 29 2007 EXTENSIONS a(32)-a(63) from Bernard Schott, Apr 19 2022 a(54) corrected by Andrey Zabolotskiy, Feb 02 2023 STATUS approved

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Last modified March 24 04:28 EDT 2023. Contains 361454 sequences. (Running on oeis4.)