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A258797
a(n) = [x^n] Product_{k=1..n} (1+x^k)^2 / x^k.
3
1, 1, 2, 6, 16, 51, 166, 554, 1896, 6595, 23212, 82582, 296393, 1071738, 3900696, 14278074, 52526972, 194108087, 720197524, 2681854490, 10019539112, 37545876368, 141080872362, 531457445806, 2006678785762, 7593123695669, 28789152013570, 109356019134584
OFFSET
0,3
FORMULA
a(n) ~ sqrt(3) * 4^n / (sqrt(Pi) * n^(3/2)).
MATHEMATICA
Table[SeriesCoefficient[Product[(1+x^k)^2/x^k, {k, 1, n}], {x, 0, n}], {n, 0, 30}]
Table[SeriesCoefficient[Product[1+x^k, {k, 1, n}]^2, {x, 0, n*(n+3)/2}], {n, 0, 30}]
CROSSREFS
Sequence in context: A100664 A317094 A339844 * A360232 A214983 A367389
KEYWORD
nonn
AUTHOR
Vaclav Kotesovec, Jun 10 2015
STATUS
approved