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 A027623 a(0) = 1; for n > 0, a(n) = number of rings with n elements. 23
 1, 1, 2, 2, 11, 2, 4, 2, 52, 11, 4, 2, 22, 2, 4, 4, 390, 2, 22, 2, 22, 4, 4, 2, 104, 11, 4, 59, 22, 2, 8, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Here a ring means (R,+,*): (R,+) is an abelian group, * is associative, a*(b+c) = a*b + a*c, (a+b)*c = a*c + b*c. Need not contain "1", * need not be commutative. The sequence continues a(32) = ? (>18590), a(33) = 4, 4, 4, 121, 2, 4, 4, 104, 2, 8, 2, 22, 22, 4, 2, 780, 11, 22, 4, 22, 2, 118, 4, 104, 4, 4, 2, 44, 2, 4, 22 = a(63), a(64) = ? (> 829826). - Christof Noebauer (christof.noebauer(AT)algebra.uni-linz.ac.at), Sep 29 2000 The paper by Antipkin/Elizarov also gives the number a(p^3) of rings of order p^3. - Hans H. Storrer (storrer(AT)math.unizh.ch), Sep 16 2003 If n is a squared prime, there are 11 mutually nonisomorphic rings of order n [see Raghavendran, p. 228]. - R. J. Mathar, Apr 20 2008 LINKS Table of n, a(n) for n=0..31. V. G. Antipkin and V. P. Elizarov, Rings of order p^3, Sib. Math. J. vol 23 no 4 (1982) pp 457-464, MR0668331 (84d:16025). R. Ballieu [ Math. Rev. 0022841; see also Math. Rev. 51#5655] showed a(8) = 52, a(p^3) = 3p + 50 if p is an odd prime. Grigore Călugăreanu, Rings with very few nilpotents, An. Sţiinţ. Univ. Al. I. Cuza Iaşi. Mat. (2018), p. 149. C. R. Fletcher, Rings of small order, Math. Gaz. vol. 64 (1980) p. 13, 1980, see esp. p. 21. Yang-Hui He and Minhyong Kim, Learning Algebraic Structures: Preliminary Investigations, arXiv:1905.02263 [cs.LG], 2019. A. V. Lelechenko, Parity of the number of primes in a given interval and algorithms of the sublinear summation, arXiv preprint arXiv:1305.1639 [math.NT], 2013. Desmond MacHale, Are There More Finite Rings than Finite Groups, Amer. Math. Monthly (2020) Vol. 127, Issue 10, 936-938. C. Noebauer, The Numbers of Small Rings. C. Noebauer, Thesis on the enumeration of near-rings. Christof Noebauer, The Numbers of Small Rings (PostScript). R. Raghavendran, Finite associative rings, Compositio Mathematica vol 21 no 2 (1969) pp. 195-229. Eric Weisstein's World of Mathematics, Ring. Index to sequences related to rings. EXAMPLE The 11 rings of order 4 (from Christian G. Bower): over C4: 1*1 = 0, 1 or 2; over C2 X C2 = <1> X <2>: (1*1,1*2,2*1,2*2) = 0000, 0001, 0002, 0012, 0102, 0112, 1002 or 1223. PROG (PARI) apply( A027623(n, e=0)=if( !e, vecprod([call(self(), f) | f <- factor(n)~]), e<3, [2^(n>0), 11][e], e==3, if(n>2, 3*sqrtnint(n, 3), 2)+50, n>2 || e>4, /*error*/("not yet implemented"), 390), [0..63]) \\ M. F. Hasler, Jan 05 2021 CROSSREFS From Bernard Schott, Mar 28 2021: (Start) -------------------------------------------------------------------- | Rings with | with 1 | without 1 | with 1 or | | n elements | | | without 1 | -------------------------------------------------------------------- | Commutative | A127707 | A342375 | A037289 | -------------------------------------------------------------------- | Noncommutative | A127708 | A342376 | A209401 | -------------------------------------------------------------------- | Commutative or | A037291 | A342377 | this sequence: a(0) = 1 | | noncommutative | | | A037234 with a(0) = 0 | -------------------------------------------------------------------- (End) Sequence in context: A236369 A001038 A283454 * A037234 A141651 A359425 Adjacent sequences: A027620 A027621 A027622 * A027624 A027625 A027626 KEYWORD nonn,nice,hard,more,mult AUTHOR N. J. A. Sloane, R. K. Guy EXTENSIONS More terms from Christian G. Bower, Jun 15 1998 a(16) from Christof Noebauer (christof.noebauer(AT)algebra.uni-linz.ac.at), Sep 29 2000 STATUS approved

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