a(1)=1, a(n) = (p-1)*a(n-1), if n is even, otherwise a(n) = p*a(n-2), where p = 5.

Terms

a(0) =1a(1) =4a(2) =5a(3) =20a(4) =25a(5) =100a(6) =125a(7) =500a(8) =625a(9) =2500a(10) =3125a(11) =12500a(12) =15625a(13) =62500a(14) =78125a(15) =312500a(16) =390625a(17) =1562500a(18) =1953125a(19) =7812500a(20) =9765625a(21) =39062500a(22) =48828125a(23) =195312500a(24) =244140625a(25) =976562500a(26) =1220703125a(27) =4882812500a(28) =6103515625a(29) =24414062500