Powerful numbers (A001694) made of nontrivial runs of identical digits.

Terms

a(0) =7744a(1) =665500a(2) =774400a(3) =5556600a(4) =6655000a(5) =7744000a(6) =55566000a(7) =66550000a(8) =77440000a(9) =447722888a(10) =448855888a(11) =555660000a(12) =665500000a(13) =774400000a(14) =1155775544a(15) =5500002244a(16) =5511005500a(17) =5556600000a(18) =6655000000a(19) =7744000000a(20) =11000004488a(21) =11122233444a(22) =22244466888