The (2^n+1)-th triangular number (cf. A000217).

Terms

a(0) =3a(1) =6a(2) =15a(3) =45a(4) =153a(5) =561a(6) =2145a(7) =8385a(8) =33153a(9) =131841a(10) =525825a(11) =2100225a(12) =8394753a(13) =33566721a(14) =134242305a(15) =536920065a(16) =2147581953a(17) =8590131201a(18) =34360131585a(19) =137439739905a(20) =549757386753