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 A000786 Number of inequivalent planar partitions of n, when considering them as 3D objects. (Formerly M1020 N0383) 11
 1, 1, 1, 2, 4, 6, 11, 19, 33, 55, 95, 158, 267, 442, 731, 1193, 1947, 3137, 5039, 8026, 12726, 20024, 31373, 48835, 75673, 116606, 178889, 273061, 415086, 628115, 946723, 1421082, 2125207, 3166152, 4700564, 6954151, 10254486, 15071903 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Partitions that are the same when regarded as 3-D objects are counted only once. - Wouter Meeussen, May 2006 REFERENCES P. A. MacMahon, Combinatory Analysis. Cambridge Univ. Press, London and New York, Vol. 1, 1915 and Vol. 2, 1916; see vol. 2, p 332. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Jean-François Alcover, Table of n, a(n) for n = 0..150 P. A. MacMahon, Combinatory analysis. Eric Weisstein's World of Mathematics, Macdonald's Plane Partition Conjecture. Eric Weisstein's World of Mathematics, Plane Partition. FORMULA Equals A000784 + A000785 + A048141 + A048142. Equals (A048141 + 3*A048140 - A000219 + 2*A048142)/3. - Wouter Meeussen, May 2006 EXAMPLE From M. F. Hasler, Oct 01 2018: (Start) For n = 2, all three plane partitions , [1 1] and [1; 1] (where ";" means next row) correspond to a 1 X 1 X 2 rectangular cuboid, therefore a(2) = 1. For n = 3, we have  ~ [1 1 1] ~ [1; 1; 1] all corresponding to a 1 X 1 X 3 rectangular cuboid or tower of height 3, and [2 1] ~ [2; 1] ~ [1 1; 1] correspond to an L-shaped object, therefore a(3) = 2. For n = 4,  ~ [1 1 1 1] ~ [1; 1; 1; 1] correspond to the 4-tower; [3 1] ~ [3; 1] ~ [2 1 1] ~ [2; 1; 1] ~ [1 1 1; 1] ~ [1 1; 1; 1] all correspond to the same L-shaped object, [2 2] ~ [2; 2] ~ [1 1; 1 1] represent a "flat" square, and it remains [2, 1; 1], so a(4) = 4. For n = 5, we again have the tower  ~ [1 1 1 1 1] ~ [1; 1; 1; 1; 1], a "narrow L" or 4-tower with one "foot" [4 1] ~ [4; 1] ~ [2 1 1 1] ~ [2; 1; 1; 1] ~ [1 1 1 1; 1] ~ [1 1; 1; 1; 1], a symmetric L-shape [3 1 1] ~ [3; 1; 1] ~ [1 1 1; 1; 1], a 3-tower with 2 feet [3 1; 1] ~ [2 1; 1; 1] ~ [2 1 1; 1], a flat 2+3 shape [3 2] ~ [3; 2] ~ [2 2 1] ~ [2; 2; 1] ~ [1 1 1; 1 1] ~ [1 1; 1 1; 1] and a 2X2 square with a cube on top, [2 1;1 1] ~ [2 2; 1] ~ [2 1; 2]. This yields a(5) = 6 classes. (End) MATHEMATICA nmax = 150; a219 = 1; a219[n_] := a219[n] = Sum[a219[n - j] DivisorSigma[2, j], {j, n}]/n; s = Product[1/(1 - x^(2i - 1))/(1 - x^(2i))^Floor[i/2], {i, 1, Ceiling[( nmax+1)/2]}] + O[x]^(nmax+1); A005987 = CoefficientList[s, x]; a048140[n_] := (a219[n] + A005987[[n+1]])/2; A048141 = Cases[Import["https://oeis.org/A048141/b048141.txt", "Table"], {_, _}][[All, 2]]; A048142 = Cases[Import["https://oeis.org/A048142/b048142.txt", "Table"], {_, _}][[All, 2]]; a = 1; a[n_] := (A048141[[n]] + 3 a048140[n] - a219[n] + 2 A048142[[n]])/3; a /@ Range[0, nmax] (* Jean-François Alcover, Dec 28 2019 *) CROSSREFS Cf. A000784, A000785, A000219, A005987, A048142, A051056-A051061, A096419. Sequence in context: A136424 A116732 A048239 * A289080 A000694 A164137 Adjacent sequences: A000783 A000784 A000785 * A000787 A000788 A000789 KEYWORD nonn,easy,nice AUTHOR N. J. A. Sloane EXTENSIONS More terms from Wouter Meeussen, 1999 Name & links edited and a(0) = 1 added by M. F. Hasler, Sep 30 2018 STATUS approved

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Last modified October 4 15:42 EDT 2023. Contains 365885 sequences. (Running on oeis4.)