a(n) = n*3^n*(4*n - 1)/9.

Terms

a(0) =0a(1) =1a(2) =14a(3) =99a(4) =540a(5) =2565a(6) =11178a(7) =45927a(8) =180792a(9) =688905a(10) =2558790a(11) =9310059a(12) =33303636a(13) =117448461a(14) =409209570a(15) =1410975855a(16) =4821232752a(17) =16343405073a(18) =55013709438a(19) =184024732275a(20) =612124372620