OFFSET
1,2
PROG
(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
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
a(19) corrected and more terms from Sean A. Irvine, Sep 07 2019
STATUS
approved