Constant term of the reduction by x^2->x+1 of the polynomial p(n,x)=1+x^n+x^(2n+1).

Terms

a(0) =2a(1) =5a(2) =10a(3) =24a(4) =59a(5) =150a(6) =386a(7) =1001a(8) =2606a(9) =6800a(10) =17767a(11) =46458a(12) =121538a(13) =318045a(14) =832418a(15) =2178920a(16) =5703875a(17) =14931950a(18) =39090754a(19) =102338337a(20) =267921062a(21) =701419680a(22) =1836329615a(23) =4807555634a(24) =12586315394a(25) =32951355125a(26) =86267692666a(27) =225851630136