G.f. satisfies: A(x) = Product_{n>=0} (1 + x*(x+x^2)^n)^2/(1 - x*(x+x^2)^n)^2.

Terms

a(0) =1a(1) =4a(2) =12a(3) =36a(4) =100a(5) =264a(6) =676a(7) =1684a(8) =4096a(9) =9764a(10) =22888a(11) =52872a(12) =120540a(13) =271600a(14) =605556a(15) =1337320a(16) =2927720a(17) =6358432a(18) =13707916a(19) =29351536a(20) =62450468a(21) =132090356a(22) =277845120a(23) =581405140a(24) =1210688864a(25) =2509483020a(26) =5178969644a(27) =10644112012a(28) =21790816340a(29) =44444609044