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A139468
a(n) = Sum{k=0..n} C(n,3k+1)^2.
3
0, 1, 4, 9, 17, 50, 261, 1275, 5028, 17253, 58601, 218042, 876789, 3537847, 13783018, 52301709, 198627921, 767778786, 3010327497, 11824753551, 46200429186, 179787741723, 700285942731, 2738134757118, 10739885115573, 42164261091351, 165467386466802
OFFSET
0,3
COMMENTS
The recurrence is same as for A119363. - Vaclav Kotesovec, Mar 12 2019
LINKS
FORMULA
a(n) ~ 4^n / (3*sqrt(Pi*n)). - Vaclav Kotesovec, Mar 12 2019
MATHEMATICA
Table[Sum[Binomial[n, 3k+1]^2, {k, 0, n}], {n, 0, 30}] (* Harvey P. Dale, Sep 08 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 12 2008
STATUS
approved