a(n) = 2*4^n+4*3^n-2^n.

Terms

a(0) =5a(1) =18a(2) =64a(3) =228a(4) =820a(5) =2988a(6) =11044a(7) =41388a(8) =157060a(9) =602508a(10) =2332324a(11) =9095148a(12) =35676100a(13) =140586828a(14) =555986404a(15) =2204846508a(16) =8762055940a(17) =34876167948a(18) =138988373284a(19) =554404335468