a(n) = A026998(2n+1, n+4).

Terms

a(0) =1a(1) =53a(2) =634a(3) =4201a(4) =20120a(5) =78753a(6) =269829a(7) =844702a(8) =2486178a(9) =7017354a(10) =19260116a(11) =51903794a(12) =138254821a(13) =365619439a(14) =962704734a(15) =2528441803a(16) =6631057180a(17) =17376467099a(18) =45514392201a(19) =119188310928a(20) =312079208726a(21) =817086876180