a(n) = (-1 + 5*2^(2*n + 1) - 3*n)/9.

Terms

a(0) =1a(1) =4a(2) =17a(3) =70a(4) =283a(5) =1136a(6) =4549a(7) =18202a(8) =72815a(9) =291268a(10) =1165081a(11) =4660334a(12) =18641347a(13) =74565400a(14) =298261613a(15) =1193046466a(16) =4772185879a(17) =19088743532a(18) =76354974145a(19) =305419896598