G.f.: Sum_{n>=0} x^n * Sum_{k=0..n} binomial(n,k)^3 * x^(2*k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =9a(5) =28a(6) =66a(7) =153a(8) =433a(9) =1345a(10) =3952a(11) =10991a(12) =30954a(13) =90988a(14) =271845a(15) =804153a(16) =2361457a(17) =6979690a(18) =20842285a(19) =62493914a(20) =187274712a(21) =561448399a(22) =1688263179a(23) =5093148285a(24) =15393417178a(25) =46570446829a(26) =141063389488a(27) =427979185898