A025569 T(2n-1,n), where T is the array defined in A025564.

1, 4, 22, 120, 665, 3732, 21153, 120835, 694590, 4013088, 23284424, 135580865, 791862354, 4637009300, 27215510265, 160053426720, 942939231825, 5564024727096, 32878367902380, 194530139772844, 1152303896900338, 6832908809693304, 40556775927067299, 240938382783554100

Offset: 1

Author

Clark Kimberling

Keywords

• nonn

Crossrefs

Cf. A025564.

Programs

PARI

T(n, k) = if( k<0 || k>2*n, 0, if(n==0, 1, polcoeff( (1 + x + x^2)^n, k)+ polcoeff( (1 + x + x^2)^(n-1), k-1)));
a(n) = T(2*n, n+1); \\ Michel Marcus, Sep 08 2019

Extensions

a(19) corrected and more terms from Sean A. Irvine, Sep 07 2019