a(1) = 5, a(n) = sigma(a(n-1)).

Terms

a(0) =5a(1) =6a(2) =12a(3) =28a(4) =56a(5) =120a(6) =360a(7) =1170a(8) =3276a(9) =10192a(10) =24738a(11) =61440a(12) =196584a(13) =491520a(14) =1572840a(15) =5433480a(16) =20180160a(17) =94859856a(18) =355532800a(19) =1040179456a(20) =2143289344a(21) =4966055344a(22) =10092086208a(23) =31800637440a(24) =137371852800a(25) =641012414823