a(n) = 6 * Sum_{m=0..n} 5^m.

Terms

a(0) =6a(1) =36a(2) =186a(3) =936a(4) =4686a(5) =23436a(6) =117186a(7) =585936a(8) =2929686a(9) =14648436a(10) =73242186a(11) =366210936a(12) =1831054686a(13) =9155273436a(14) =45776367186a(15) =228881835936