G.f.: Product_{n=-oo..+oo} (1 + x^n*(1 + x^n)^n).

Terms

a(0) =4a(1) =2a(2) =12a(3) =6a(4) =20a(5) =14a(6) =62a(7) =46a(8) =108a(9) =74a(10) =244a(11) =176a(12) =486a(13) =364a(14) =956a(15) =680a(16) =1682a(17) =1326a(18) =3300a(19) =2522a(20) =5884a(21) =4562a(22) =10328a(23) =8312a(24) =18680a(25) =15028a(26) =31472a(27) =25832a(28) =54888a(29) =45676