a(n) = n*3^(n-4).

Terms

a(0) =1a(1) =4a(2) =15a(3) =54a(4) =189a(5) =648a(6) =2187a(7) =7290a(8) =24057a(9) =78732a(10) =255879a(11) =826686a(12) =2657205a(13) =8503056a(14) =27103491a(15) =86093442a(16) =272629233a(17) =860934420a(18) =2711943423a(19) =8523250758a(20) =26732013741a(21) =83682825624a(22) =261508830075a(23) =815907549834