a(1) = 2, a(n) = sigma(a(n-1)).

Terms

a(0) =2a(1) =3a(2) =4a(3) =7a(4) =8a(5) =15a(6) =24a(7) =60a(8) =168a(9) =480a(10) =1512a(11) =4800a(12) =15748a(13) =28672a(14) =65528a(15) =122880a(16) =393192a(17) =1098240a(18) =4124736a(19) =15605760a(20) =50328576a(21) =149873152a(22) =371226240a(23) =1710858240a(24) =7926750720a(25) =33463001088a(26) =109760857440a(27) =384120963072