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 A380985 #25 Feb 23 2025 13:43:44 %S A380985 1,20,110,2100,20100,200100,2000100,20000100,200000100,2000000100, %T A380985 20000000100,200000000100,2000000000100,20000000000100, %U A380985 200000000000100,2000000000000100,20000000000000100,200000000000000100,2000000000000000100,20000000000000000100 %N A380985 Numbers whose k-th digit indicates the number of digits which occur k times. %C A380985 The most significant digit is digit position k=1, meaning that it counts how many numbers appear 1 time. %F A380985 a(n) = 2*10^(n-1) + 100 for n >= 4. - _Andrew Howroyd_, Feb 22 2025 %e A380985 110 is a term because 1 number appears once (0), 1 number appears twice (1) and 0 numbers appear 3 times. %o A380985 (Python) %o A380985 from collections import Counter %o A380985 def ok(n): %o A380985 d = list(map(int, str(n))) %o A380985 c = Counter(Counter(d).values()) %o A380985 return all(dk == c[k] for k, dk in enumerate(d, 1)) # _Michael S. Branicky_, Feb 18 2025 %Y A380985 Cf. A046043. %K A380985 nonn,easy,base %O A380985 1,2 %A A380985 _Leo Crabbe_, Feb 11 2025