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 A381258 #20 Mar 05 2025 12:06:33 %S A381258 0,1,135,5733,11229,42705,50445,117649,131365,168093,636405,699825, %T A381258 1269495,2528155,4226175,6176709,6502545,9365265,9551115,13227021, %U A381258 14464485,14912625,20859435,26903605,28251265,30589905,32660901,37597329,41506875,42766465,55452075,56192535,111898605 %N A381258 Numbers k such that 7*k+1 divides 7^k+1. %C A381258 The numbers are called Curzon numbers by Tattersall (p. 85, exercise 43). %D A381258 James J. Tattersall, Elementary Number Theory in Nine Chapters, Second Edition, Cambridge University Press, 2005, p. 85. %H A381258 Giovanni Resta, <a href="https://www.numbersaplenty.com/set/Curzon_number/">Curzon numbers</a>, Numbers Aplenty. %t A381258 Select[Range[0,10^7],PowerMod[7,#,7#+1]==7#&] (* _James C. McMahon_, Mar 05 2025 *) %o A381258 (PARI) isok(n) = my(m=7*n+1); Mod(7, m)^n==-1 %Y A381258 Cf. A224486, A222948, A230076, A381256, A381257. %K A381258 nonn %O A381258 1,3 %A A381258 _René-Louis Clerc_, Feb 18 2025