a(n) = sigma_3(3n+1).

Terms

a(0) =1a(1) =73a(2) =344a(3) =1134a(4) =2198a(5) =4681a(6) =6860a(7) =11988a(8) =15751a(9) =25112a(10) =29792a(11) =44226a(12) =50654a(13) =73710a(14) =79508a(15) =109512a(16) =117993a(17) =160454a(18) =167832a(19) =219510a(20) =226982a(21) =299593a(22) =300764a(23) =390096a(24) =389018a(25) =500780a(26) =493040a(27) =620298a(28) =619164a(29) =779220