a(n) = (2^n+1)*(2^n+2)/6.

Terms

a(0) =1a(1) =2a(2) =5a(3) =15a(4) =51a(5) =187a(6) =715a(7) =2795a(8) =11051a(9) =43947a(10) =175275a(11) =700075a(12) =2798251a(13) =11188907a(14) =44747435a(15) =178973355a(16) =715860651a(17) =2863377067a(18) =11453377195a(19) =45813246635a(20) =183252462251a(21) =733008800427