a(n) is the constant term in expansion of Product_{k=1..n} (x^(k^2) + 1/x^(k^2))^2.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =16a(5) =40a(6) =88a(7) =222a(8) =570a(9) =1564a(10) =4516a(11) =13874a(12) =41866a(13) =137432a(14) =442964a(15) =1492610a(16) =4998674a(17) =17204844a(18) =59175316a(19) =207299554a(20) =727137516a(21) =2582078416a(22) =9179001124a(23) =32943918428a(24) =118453240846a(25) =428937325964