Expansion of g.f. (1-2*x)/(1-7*x).

Terms

a(0) =1a(1) =5a(2) =35a(3) =245a(4) =1715a(5) =12005a(6) =84035a(7) =588245a(8) =4117715a(9) =28824005a(10) =201768035a(11) =1412376245a(12) =9886633715a(13) =69206436005a(14) =484445052035