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 A135580 #12 Apr 16 2025 19:40:15 %S A135580 3,13,23,3,43,53,3,73,83,3,103,113,23,13,43,53,163,173,83,193,3,13, %T A135580 223,233,43,53,263,73,283,293,3,313,23,3,43,353,3,373,383,3,3,41,23, %U A135580 433,443,53,463,73,83,3,503,13,523,53,43,53,563,73,83,593,3,613,23,3,643,653,3 %N A135580 Largest prime visible as a substring of 10n+3. %F A135580 a(n) = A047814(A017305(n)). - _Michel Marcus_, Apr 16 2025 %e A135580 a(0) = 3 because 10 * 0 + 3 = 3 (prime), %e A135580 a(1) = 13 because 10 * 1 + 3 = 13 (prime), %e A135580 a(3) = 3 because 10 * 3 + 3 = 33 (composite). Substrings of 33 are 0, 3, 33 and the largest prime of these is 3. %e A135580 a(78) = 83 because 10 * 78 + 3 = 783 (composite). The largest prime substring is 83. %t A135580 a[n_]:=Max[Select[FromDigits/@Subsequences[IntegerDigits[10n+3]],PrimeQ]] (* _James C. McMahon_, Apr 16 2025 *) %Y A135580 Cf. A017305, A030431, A047814. %K A135580 nonn,base %O A135580 0,1 %A A135580 _Zak Seidov_, Feb 24 2008