Coefficient of x^n in the expansion of (1+x+x^3)^(2*n).

Terms

a(0) =1a(1) =2a(2) =6a(3) =26a(4) =126a(5) =612a(6) =2970a(7) =14534a(8) =71838a(9) =357884a(10) =1793296a(11) =9026976a(12) =45612450a(13) =231224060a(14) =1175422590a(15) =5989693176a(16) =30586693182a(17) =156483812892a(18) =801908994852