a(n) = 5^n-(-1)^n.

Terms

a(0) =0a(1) =6a(2) =24a(3) =126a(4) =624a(5) =3126a(6) =15624a(7) =78126a(8) =390624a(9) =1953126a(10) =9765624a(11) =48828126a(12) =244140624a(13) =1220703126a(14) =6103515624a(15) =30517578126a(16) =152587890624a(17) =762939453126