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

Terms

a(0) =1a(1) =1a(2) =1a(3) =10a(4) =49a(5) =151a(6) =901a(7) =5881a(8) =28225a(9) =165565a(10) =1158601a(11) =6993196a(12) =44201521a(13) =320103070a(14) =2200745821a(15) =15118248601a(16) =113390231809a(17) =845797019077