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with(combinat):a:=n->sum(sum(numbcomp(2*n, n), j=2..n), k=1..n): seq(a(n), n=2..21); # Zerinvary Lajos, May 10 2007
a(n) = 6* * A002802(n). - Zerinvary Lajos, Jun 02 2007
Sum_{n>=0} 1/a(n) = 2 - Pi/sqrt(3). - _) = 2 - A093602. - _Amiram Eldar_, Oct 13 2020
Cf. A000217, A000984, A001801, A002802, A051133, A086466, A093602.
a(n) = 6*A002802(n). - Zerinvary Lajos, Jun 02 2007
a(n) = 2 * A051133(n+1).
Equals 2 * A051133(n+1). Cf. A001801.
Cf. A001801.
Sum_{n>=0} 1/a(n) = 2 - Pi/sqrt(3). - Amiram Eldar, Oct 13 2020
From _Ilya Gutkovskiy, _, Jan 31 2017: (Start)
