a(n) = T(0,n) + T(1,n-1) + ... + T(n,0), array T given by A048472.

Terms

a(0) =1a(1) =3a(2) =9a(3) =27a(4) =77a(5) =207a(6) =529a(7) =1299a(8) =3093a(9) =7191a(10) =16409a(11) =36891a(12) =81949a(13) =180255a(14) =393249a(15) =852003a(16) =1835045a(17) =3932199a(18) =8388649a(19) =17825835a(20) =37748781a(21) =79691823a(22) =167772209a(23) =352321587a(24) =738197557a(25) =1543503927a(26) =3221225529a(27) =6710886459a(28) =13958643773a(29) =28991029311