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

Terms

a(0) =1a(1) =4a(2) =17a(3) =73a(4) =311a(5) =1309a(6) =5447a(7) =22453a(8) =91871a(9) =373789a(10) =1514327a(11) =6115333a(12) =24636431a(13) =99073069a(14) =397878407a(15) =1596280213a(16) =6399436991a(17) =25640729149a(18) =102691925687a(19) =411154861093