a(n) = 5^(n/2) for n even, a(n) = 3*5^((n-1)/2) for n odd.

Terms

a(0) =1a(1) =3a(2) =5a(3) =15a(4) =25a(5) =75a(6) =125a(7) =375a(8) =625a(9) =1875a(10) =3125a(11) =9375a(12) =15625a(13) =46875a(14) =78125a(15) =234375a(16) =390625a(17) =1171875a(18) =1953125a(19) =5859375a(20) =9765625a(21) =29296875a(22) =48828125a(23) =146484375a(24) =244140625a(25) =732421875a(26) =1220703125a(27) =3662109375a(28) =6103515625a(29) =18310546875