a(n) = 2*5^n-1.

Terms

a(0) =1a(1) =9a(2) =49a(3) =249a(4) =1249a(5) =6249a(6) =31249a(7) =156249a(8) =781249a(9) =3906249a(10) =19531249a(11) =97656249a(12) =488281249a(13) =2441406249a(14) =12207031249a(15) =61035156249a(16) =305175781249