a(n) = Sum_{k=0..n} C(n,k)*sigma(n+k) for n>=1.

Terms

a(0) =4a(1) =18a(2) =55a(3) =150a(4) =379a(5) =915a(6) =2146a(7) =4934a(8) =11080a(9) =24833a(10) =54476a(11) =119091a(12) =259432a(13) =556700a(14) =1195135a(15) =2561094a(16) =5428597a(17) =11488866a(18) =24350993a(19) =51296325a(20) =107427025a(21) =225330244a(22) =472762497a(23) =985966379a(24) =2049357779a(25) =4267962522a(26) =8887535983a(27) =18431783744