a(n)=(5/3)*(1+2*(-5)^(n-1)).

Terms

a(0) =1a(1) =5a(2) =-15a(3) =85a(4) =-415a(5) =2085a(6) =-10415a(7) =52085a(8) =-260415a(9) =1302085a(10) =-6510415a(11) =32552085a(12) =-162760415a(13) =813802085a(14) =-4069010415a(15) =20345052085a(16) =-101725260415a(17) =508626302085