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 A383613 #13 May 09 2025 23:12:48 %S A383613 -1,3,-1,-1,17,-1,3,43,997,3701,3,31,607,2837,-1,3,11,929,5843,57349, %T A383613 -1,5,11,-1,4447,31063,224813,-1,5,-1,277,2477,77377,292223,9999991, %U A383613 65442077,7,11,809,7019,24379,262433,9862243,61879669,-1,-1,11,499,1571,17669,342281,1303613,32685743,763137931,-1 %N A383613 Square array read by antidiagonals upwards: T(n,k) (for n>1 and k>0) is the smallest k-digit prime p such that prevprime(p) appears as a substring in p^n; or -1 if no such prime exists. %e A383613 T(2,4) = 3701, because prevprime(3701) = 3697 is a substring of 3701^2 = 13697401, and no smaller 4-digit prime satisfies this condition. %e A383613 Top left corner begins at T(2,1): %e A383613 -1, -1, -1, 3701, -1, ... %e A383613 3, 17, 997, 2837, 57349, ... %e A383613 -1, 43, 607, 5843, 31063, ... %e A383613 3, 31, 929, 4447, 77377, ... %e A383613 . .., ..., ...., ....., ... %o A383613 (PARI) T(n,k) = forprime(p=10^(k-1), 10^k-1, if (#strsplit(Str(p^n), Str(precprime(p-1))) >= 2, return(p));); return(-1); \\ _Michel Marcus_, May 02 2025 %Y A383613 Cf. A381969. %K A383613 sign,tabl,base %O A383613 2,2 %A A383613 _Jean-Marc Rebert_, May 02 2025