G.f.: Sum_{n>=0} x^n * Sum_{k=0..n} binomial(n,k)^2 * x^k/(1-2*x)^k.

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =23a(5) =72a(6) =227a(7) =725a(8) =2332a(9) =7537a(10) =24465a(11) =79720a(12) =260633a(13) =854539a(14) =2808768a(15) =9252315a(16) =30536925a(17) =100959558a(18) =334301159a(19) =1108483583a(20) =3680134756a(21) =12231824111a(22) =40697552035a(23) =135536687436a(24) =451776392011