a(n) = [x^n] (1 + theta_3(x))^n/(2^n*(1 - x)), where theta_3() is the Jacobi theta function.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =20a(5) =57a(6) =160a(7) =422a(8) =1076a(9) =2780a(10) =7449a(11) =20462a(12) =56348a(13) =153909a(14) =418268a(15) =1139703a(16) =3126068a(17) =8618611a(18) =23801146a(19) =65708424a(20) =181391905a(21) =501296216a(22) =1387834518a(23) =3848187985a(24) =10680579812a(25) =29660831057a(26) =82415406493a(27) =229156296047a(28) =637659848888