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

Terms

a(0) =0a(1) =1a(2) =15a(3) =150a(4) =1250a(5) =9375a(6) =65625a(7) =437500a(8) =2812500a(9) =17578125a(10) =107421875a(11) =644531250a(12) =3808593750a(13) =22216796875a(14) =128173828125a(15) =732421875000