O.g.f.: exp( Sum_{n>=1} (sigma(2*n^2)-sigma(n^2)) * x^n/n ).

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =42a(5) =102a(6) =238a(7) =522a(8) =1130a(9) =2394a(10) =4926a(11) =9978a(12) =19890a(13) =38942a(14) =75254a(15) =143598a(16) =270506a(17) =504126a(18) =929926a(19) =1698322a(20) =3074010a(21) =5516898a(22) =9820550a(23) =17349554a(24) =30430610a(25) =53007162a(26) =91734262a(27) =157771538a(28) =269734714a(29) =458542822