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 A100909 #15 Jul 16 2025 17:52:58 %S A100909 1000000000,100000000,10000000,1000000,100000,10000,1000,100,10,1, %T A100909 1100000000,200000000,110000000,101000000,100100000,100010000, %U A100909 100001000,100000100,100000010,100000001,1010000000,110000000,20000000,11000000,10100000,10010000,10001000 %N A100909 Concatenate number of occurrences in n of each decimal digit from 0 to 9 and drop leading zeros. %C A100909 n = 0 is normally represented as the single digit 0, so a(0) = 1000000000. This representation system is inherently ambiguous by disregarding the order of n's digits but without modification will correctly identify those digits for all numbers up to 999999999 decimal; i.e., a(999999999) = 9; and for many beyond (e.g., a(121212121212121212) = a(111222111222222111) = ... = 990000000). However, for any n in which more than 9 of any single digit occur, additional ambiguity is introduced unless some type of grouping is also used (say, parentheses around or bars over a group of consecutive digits when written) so that, for example, (10) is known to represent 9999999999 rather than 8. %H A100909 Mia Boudreau, <a href="/A100909/b100909.txt">Table of n, a(n) for n = 0..10000</a> %e A100909 a(12) = 110000000 as 12 consists only of one 1 and one 2, hence the following are concatenated: 0 1 1 0 0 0 0 0 0 0 and dropping the leading 0 gives 110000000 (= a(21) also). %t A100909 A100909[n_] := FromDigits[RotateRight[DigitCount[n]]]; %t A100909 Array[A100909, 25, 0] (* _Paolo Xausa_, Jul 16 2025 *) %Y A100909 Cf. A100910 (each number of digit occurrences is a separate term). %K A100909 base,easy,nonn %O A100909 0,1 %A A100909 _Rick L. Shepherd_, Nov 21 2004