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

Terms

a(0) =3a(1) =0a(2) =4a(3) =5a(4) =18a(5) =42a(6) =115a(7) =296a(8) =780a(9) =2037a(10) =5338a(11) =13970a(12) =36579a(13) =95760a(14) =250708a(15) =656357a(16) =1718370a(17) =4498746a(18) =11777875a(19) =30834872a(20) =80726748a(21) =211345365a(22) =553309354a(23) =1448582690a(24) =3792438723a(25) =9928733472a(26) =25993761700a(27) =68052551621a(28) =178163893170a(29) =466439127882