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 A086163 Number of monomial ideals in two variables x, y that are artinian, integrally closed, of colength n and contain x^4. 4
 1, 1, 2, 3, 4, 6, 7, 7, 10, 13, 13, 16, 18, 19, 23, 27, 28, 32, 36, 39, 43, 48, 50, 56, 61, 65, 71, 77, 81, 90, 95, 100, 108, 116, 121, 132, 139, 145, 156, 167, 172, 185, 194, 202, 215, 228, 235, 250, 262, 273, 287, 302, 311, 329, 343, 356, 373, 390, 402, 424, 439, 454 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Alternatively, "concave partitions" of n with at most 4 parts, where a concave partition is defined by demanding that the monomial ideal, generated by the monomials whose exponents do no lie in the Ferrers diagram of the partition, is integrally closed. REFERENCES G. E. Andrews, The Theory of Partitions, Addison-Wesley Publishing Company, 1976. M. Paulsen and J. Snellman, Enumerativa egenskaper hos konkava partitioner (in Swedish), Department of Mathematics, Stockholm University. V. Crispin Quinonez, Integrally closed monomial ideals and powers of ideals, Research Reports in Mathematics Number 7 2002, Department of Mathematics, Stockholm University LINKS Jan Snellman and Michael Paulsen, Enumeration of Concave Integer Partitions, J. Integer Seqs., Vol. 7, 2004. FORMULA G.f.: (1+t^2+t^4+t^5-t^6-t^7+2*t^9-2*t^10 -t^11 -2*t^12 + 2*t^13 -t^14 -t^15 +t^16 +t^17 +t^18-t^19)/((1-t)*(1-t^3)*(1-t^6)*(1-t^10)). MATHEMATICA CoefficientList[ Series[ (1 + t^2 + t^4 + t^5 - t^6 - t^7 + 2*t^9 - 2*t^10 - t^11 - 2*t^12 + 2*t^13 - t^14 - t^15 + t^16 + t^17 + t^18 - t^19) / ((1 - t)*(1 - t^3)*(1 - t^6)*(1 - t^10)), {t, 0, 65}], t] CROSSREFS Cf. A084913. Cf. A084913, A086162, A086163. Sequence in context: A071652 A089884 A172312 * A175059 A071789 A131870 Adjacent sequences:  A086160 A086161 A086162 * A086164 A086165 A086166 KEYWORD nonn AUTHOR Jan Snellman (Jan.Snellman(AT)math.su.se), Aug 25 2003 EXTENSIONS More terms from Robert G. Wilson v, Aug 27 2003 STATUS approved

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Last modified February 16 12:48 EST 2019. Contains 320163 sequences. (Running on oeis4.)