a(n) = 2 + Sum_{k=0..n-1} A176513(4*k+1).

Terms

a(0) =2a(1) =3a(2) =4a(3) =11a(4) =40a(5) =157a(6) =656a(7) =2721a(8) =11346a(9) =47337a(10) =197398a(11) =823451a(12) =3434718a(13) =14326815a(14) =59760224a(15) =249271079a(16) =1039759044a(17) =4337038713a(18) =18090636780a(19) =75459593981a(20) =314756746798