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

Terms

a(0) =1a(1) =1a(2) =5a(3) =13a(4) =53a(5) =176a(6) =677a(7) =2451a(8) =9333a(9) =34978a(10) =133580a(11) =508806a(12) =1953701a(13) =7509178a(14) =28981643a(15) =112046213a(16) =434289525a(17) =1686080622a(18) =6557830310a(19) =25542229740a(20) =99622788428a(21) =389023326600