a(n)=(6/7)*(2+5*(-6)^(n-1)).

Terms

a(0) =1a(1) =6a(2) =-24a(3) =156a(4) =-924a(5) =5556a(6) =-33324a(7) =199956a(8) =-1199724a(9) =7198356a(10) =-43190124a(11) =259140756a(12) =-1554844524a(13) =9329067156a(14) =-55974402924a(15) =335846417556