Coefficient of x^5 in expansion of (1 + x + x^2)^n.

Terms

a(0) =3a(1) =16a(2) =51a(3) =126a(4) =266a(5) =504a(6) =882a(7) =1452a(8) =2277a(9) =3432a(10) =5005a(11) =7098a(12) =9828a(13) =13328a(14) =17748a(15) =23256a(16) =30039a(17) =38304a(18) =48279a(19) =60214a(20) =74382a(21) =91080a(22) =110630a(23) =133380a(24) =159705a(25) =190008a(26) =224721a(27) =264306a(28) =309256a(29) =360096