a(n) = T(2n-1, n-1), T given by A026780.

Terms

a(0) =1a(1) =5a(2) =24a(3) =117a(4) =580a(5) =2916a(6) =14834a(7) =76221a(8) =395048a(9) =2063104a(10) =10847078a(11) =57373672a(12) =305110106a(13) =1630489090a(14) =8751851866a(15) =47166202181a(16) =255128842340