a(n) = sigma(2*a(n-1)) for n>1 with a(1)=1.

Terms

a(0) =1a(1) =3a(2) =12a(3) =60a(4) =360a(5) =2418a(6) =12544a(7) =58311a(8) =299520a(9) =2235324a(10) =14515200a(11) =122882760a(12) =1152592896a(13) =8365593600a(14) =81041688000a(15) =812657664000