G.f.: sqrt( 1 + Sum_{n>=1} 4*x^(n^2) ).

Terms

a(0) =1a(1) =2a(2) =-2a(3) =4a(4) =-8a(5) =24a(6) =-72a(7) =224a(8) =-720a(9) =2370a(10) =-7940a(11) =27020a(12) =-93128a(13) =324424a(14) =-1140512a(15) =4041024a(16) =-14416030a(17) =51737020a(18) =-186663414a(19) =676656868a(20) =-2463300660a(21) =9001715448a(22) =-33009329056a(23) =121427520528a(24) =-447970371792