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

Terms

a(0) =2a(1) =5a(2) =11a(3) =26a(4) =63a(5) =157a(6) =398a(7) =1021a(8) =2639a(9) =6854a(10) =17855a(11) =46601a(12) =121770a(13) =318421a(14) =833027a(15) =2179906a(16) =5705471a(17) =14934533a(18) =39094934a(19) =102345101a(20) =267932007a(21) =701437390a(22) =1836358271a(23) =4807602001a(24) =12586390418a(25) =32951476517a(26) =86267889083a(27) =225851947946