a(n) = T(2n+1,n+2), T given by A026736.

Terms

a(0) =1a(1) =5a(2) =21a(3) =85a(4) =342a(5) =1380a(6) =5598a(7) =22836a(8) =93640a(9) =385734a(10) =1595232a(11) =6619374a(12) =27545269a(13) =114901685a(14) =480282369a(15) =2011058681a(16) =8433331523a(17) =35410037683a(18) =148842787215a(19) =626234799703