a(0) = 1; a(n) = Sum_{k=1..n} gcd(n,k)*a(n-k).

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =18a(5) =34a(6) =104a(7) =174a(8) =510a(9) =1082a(10) =2714a(11) =4658a(12) =16316a(13) =25634a(14) =71978a(15) =169258a(16) =419878a(17) =712386a(18) =2349160a(19) =3773934a(20) =11980874a(21) =25033510a(22) =59450390a(23) =104012646a(24) =368769280a(25) =625406094a(26) =1645569502a(27) =3654549058a(28) =9504389550a(29) =16006708782