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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Table of n, a(n) for n=0..27.

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

Cf. A258798, A258799.

Sequence in context: A100664 A317094 A339844 * A214983 A214833 A263593

Adjacent sequences:  A258794 A258795 A258796 * A258798 A258799 A258800

KEYWORD

nonn

AUTHOR

Vaclav Kotesovec, Jun 10 2015

STATUS

approved

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Last modified June 17 15:38 EDT 2021. Contains 345085 sequences. (Running on oeis4.)