First differences of A000166.

Terms

a(0) =-1a(1) =1a(2) =1a(3) =7a(4) =35a(5) =221a(6) =1589a(7) =12979a(8) =118663a(9) =1201465a(10) =13349609a(11) =161530271a(12) =2114578091a(13) =29780308117a(14) =448995414685