a(n)^3 is smallest cube containing exactly n 9's.

Terms

a(0) =9a(1) =31a(2) =99a(3) =998a(4) =999a(5) =7937a(6) =9999a(7) =99998a(8) =99999a(9) =996999a(10) =999999a(11) =6688699a(12) =9999999a(13) =97609999a(14) =99969999a(15) =999999998a(16) =999899999a(17) =9998999999a(18) =9999999999a(19) =9999699999a(20) =99999989999a(21) =99998999999a(22) =997999998999