a(1) = 1; a(n+1) = Sum_{d|n} (-1)^(n/d+1)*d*a(d).

Terms

a(0) =1a(1) =1a(2) =1a(3) =4a(4) =13a(5) =66a(6) =394a(7) =2759a(8) =22053a(9) =198481a(10) =1984746a(11) =21832207a(12) =261986098a(13) =3405819275a(14) =47681467093a(15) =715222006464