This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A255678 #12 Jan 23 2022 12:46:09 %S A255678 0,4,14,45,143,452,1431,4549,14334,46097 %N A255678 Smallest number m needing exactly n distinct digits in decimal representation of m*(m+1)/2. %C A255678 A118668(a(n)) = n and A118668(m) <> n for m < a(n). %o A255678 (Haskell) %o A255678 a255678 n = head $ filter ((== n) . a118668) [0..] %Y A255678 Cf. A118668, A260214 (the resulting triangular numbers). %K A255678 nonn,base,fini,full %O A255678 1,2 %A A255678 _Reinhard Zumkeller_, Jul 11 2015