OFFSET
1,2
MATHEMATICA
a[n_] := SeriesCoefficient[(1-x^n) Product[1-x^(2k), {k, 1, n-1}]/(1-x)^n, {x, 0, n(n-1)/2}];
Array[a, 20]
CROSSREFS
KEYWORD
nonn
AUTHOR
Jean-François Alcover, Mar 25 2020
STATUS
approved