a(n) = A026637(2*n, n-1).

Terms

a(0) =1a(1) =5a(2) =20a(3) =74a(4) =278a(5) =1049a(6) =3980a(7) =15170a(8) =58052a(9) =222914a(10) =858512a(11) =3314960a(12) =12829070a(13) =49748705a(14) =193259660a(15) =751954250a(16) =2929965020a(17) =11431262390a(18) =44651369720a(19) =174597927740a(20) =683388447260