G.f.: Product_{i>=1, j>=1, k>=1, l>=1} (1 + x^(i*j*k*l)).

Terms

a(0) =1a(1) =1a(2) =4a(3) =8a(4) =20a(5) =36a(6) =86a(7) =150a(8) =314a(9) =564a(10) =1088a(11) =1902a(12) =3557a(13) =6085a(14) =10902a(15) =18506a(16) =32124a(17) =53584a(18) =91133a(19) =149749a(20) =249315a(21) =405121a(22) =662582a(23) =1063152a(24) =1714580a(25) =2719842a(26) =4327302a(27) =6797316a(28) =10686005a(29) =16622003