a(n) = (1 + 3^n - 2*3^(n/2))/4 if n is even, (1 + 3^n - 4*3^((n-1)/2))/4 if n odd.

Terms

a(0) =0a(1) =1a(2) =4a(3) =16a(4) =52a(5) =169a(6) =520a(7) =1600a(8) =4840a(9) =14641a(10) =44044a(11) =132496a(12) =397852a(13) =1194649a(14) =3585040a(15) =10758400a(16) =32278480a(17) =96845281a(18) =290545684a(19) =871666576a(20) =2615029252a(21) =7845176329a(22) =23535617560a(23) =70607118400a(24) =211821620920a(25) =635465659921