a(n) = 1 + Sum_{k=1..n} binomial(n,k) * sigma(k).

Terms

a(0) =2a(1) =6a(2) =17a(3) =46a(4) =117a(5) =285a(6) =674a(7) =1558a(8) =3536a(9) =7911a(10) =17503a(11) =38377a(12) =83501a(13) =180480a(14) =387882a(15) =829606a(16) =1766999a(17) =3749766a(18) =7931115a(19) =16724871a(20) =35173778a(21) =73794661a(22) =154485528a(23) =322771345a(24) =673155142a(25) =1401536935a(26) =2913490376a(27) =6047714600a(28) =12536770559a(29) =25956242580