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

Terms

a(0) =1a(1) =-5a(2) =-35a(3) =-185a(4) =-935a(5) =-4685a(6) =-23435a(7) =-117185a(8) =-585935a(9) =-2929685a(10) =-14648435a(11) =-73242185a(12) =-366210935a(13) =-1831054685a(14) =-9155273435a(15) =-45776367185a(16) =-228881835935