a(n) = [x^n] Product_{j=1..n, k=1..n} (1 + x^(k^j)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =13a(5) =33a(6) =81a(7) =194a(8) =458a(9) =1074a(10) =2513a(11) =5876a(12) =13722a(13) =31961a(14) =74168a(15) =171395a(16) =394450a(17) =904393a(18) =2066770a(19) =4709538a(20) =10704306a(21) =24273709a(22) =54926658a(23) =124036675a(24) =279559571a(25) =628906790a(26) =1412254773a(27) =3165780760a(28) =7084607367a(29) =15828666526