Partial sums of A000140.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =32a(5) =133a(6) =706a(7) =4542a(8) =33770a(9) =284519a(10) =2694100a(11) =28292286a(12) =324935676a(13) =4052477864a(14) =54679031852a(15) =793359552994