a(n)=(7/4)*(1+3*(-7)^(n-1)).

Terms

a(0) =1a(1) =7a(2) =-35a(3) =259a(4) =-1799a(5) =12607a(6) =-88235a(7) =617659a(8) =-4323599a(9) =30265207a(10) =-211856435a(11) =1482995059a(12) =-10380965399a(13) =72666757807a(14) =-508667304635