a(n) = n*x^n + (n-1)*x^(n-1) + . . . + x + 1 for x=4.

Terms

a(0) =1a(1) =5a(2) =37a(3) =229a(4) =1253a(5) =6373a(6) =30949a(7) =145637a(8) =669925a(9) =3029221a(10) =13514981a(11) =59652325a(12) =260978917a(13) =1133394149a(14) =4891490533a(15) =20997617893a(16) =89717094629a(17) =381774870757