OFFSET
1,2
COMMENTS
Plane partitions seen as 3-dimensional-objects can have a mirror symmetry plane.
LINKS
Jean-François Alcover, Table of n, a(n) for n = 1..1000
EXAMPLE
n=3 gives 4 forms: {{3}}; {{1,1,1}}={{1},{1},{1}}; {{2,1}}={{2},{1}}; {{1,1},{1}}.
MATHEMATICA
terms = 100;
a219[0] = 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[ (terms+1)/2]}] + O[x]^(terms+1);
A005987 = CoefficientList[s, x];
a[n_] := (a219[n] + A005987[[n+1]])/2;
a /@ Range[terms] (* Jean-François Alcover, Dec 28 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
Edited by N. J. A. Sloane at the suggestion of Andrew S. Plewe, Jun 08 2007
STATUS
approved