Partial sums of A001764.

Terms

a(0) =1a(1) =2a(2) =5a(3) =17a(4) =72a(5) =345a(6) =1773a(7) =9525a(8) =52788a(9) =299463a(10) =1730178a(11) =10144818a(12) =60211926a(13) =361042498a(14) =2183809018a(15) =13308564682a(16) =81637319641a(17) =503667864976