a(n)^3 is smallest cube containing exactly n 7's.

Terms

a(0) =3a(1) =26a(2) =83a(3) =173a(4) =1983a(5) =2953a(6) =19753a(7) =90643a(8) =258999a(9) =426859a(10) =4255753a(11) =13955253a(12) =42111153a(13) =92356426a(14) =425851173a(15) =878398753a(16) =9197190176a(17) =9196397753a(18) =89494606688a(19) =390974932563a(20) =918856391641