a(n) = Sum{T(k,k-1)}, k = 1,2,...,n, where T is the array defined in A026082.

Terms

a(0) =1a(1) =3a(2) =6a(3) =9a(4) =22a(5) =55a(6) =153a(7) =431a(8) =1235a(9) =3555a(10) =10278a(11) =29793a(12) =86562a(13) =251983a(14) =734776a(15) =2145825a(16) =6275148a(17) =18373299a(18) =53856156a(19) =158025189a(20) =464112300a(21) =1364247183a(22) =4013353935a(23) =11815188003a(24) =34807249137a(25) =102606325139a(26) =302646363728a(27) =893175905781