a(n) = ((2n+1)*3^n - 1)/2.

Terms

a(0) =4a(1) =22a(2) =94a(3) =364a(4) =1336a(5) =4738a(6) =16402a(7) =55768a(8) =186988a(9) =620014a(10) =2037190a(11) =6643012a(12) =21523360a(13) =69353050a(14) =222408058a(15) =710270896a(16) =2259952852a(17) =7167279046a(18) =22664098606a(19) =71479080220a(20) =224897593864a(21) =706073841202