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 A027623 a(0) = 1; for n > 0, a(n) = number of rings with n elements. 13
 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 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 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 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 odd prime. C. R. Fletcher, Rings of small order, Math. Gaz. vol. 64 (1980) p. 13, 1980, see esp. p. 21. A. V. Lelechenko, Parity of the number of primes in a given interval and algorithms of the sublinear summation, arXiv preprint arXiv:1305.1639, 2013 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) p 195-229. Eric Weisstein's World of Mathematics, Ring 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. CROSSREFS Cf. A037289, A037291. Sequence in context: A001038 A283454 * A037234 A141651 A213990 A275429 Adjacent sequences:  A027620 A027621 A027622 * A027624 A027625 A027626 KEYWORD nonn,nice,hard,more,mult AUTHOR 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 Christof Noebauer also reports that 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) STATUS approved

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Last modified December 15 03:52 EST 2018. Contains 318141 sequences. (Running on oeis4.)