Expansion of g.f. (1+x)/(1-6*x).

Terms

a(0) =1a(1) =7a(2) =42a(3) =252a(4) =1512a(5) =9072a(6) =54432a(7) =326592a(8) =1959552a(9) =11757312a(10) =70543872a(11) =423263232a(12) =2539579392a(13) =15237476352a(14) =91424858112a(15) =548549148672