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 A381257 #16 Apr 02 2025 20:58:20 %S A381257 0,1,6,30,58,70,73,90,101,105,121,125,146,153,166,170,181,182,185,210, %T A381257 233,241,242,266,282,290,322,373,381,385,390,397,441,445,446,450,453, %U A381257 530,557,562,585,593,601,602,605,606,621,646,653,670,685,710,726,805,810,817,833,837,853,866 %N A381257 Numbers k such that 6*k+1 divides 6^k+1. %C A381257 The numbers are called Curzon numbers by Tattersall (p. 85, exercise 43). %D A381257 James J. Tattersall, Elementary Number Theory in Nine Chapters, Second Edition, Cambridge University Press, 2005, p. 85. %H A381257 Giovanni Resta,<a href="https://www.numbersaplenty.com/set/Curzon_number/">Curzon numbers </a>, Numbers Aplenty. %e A381257 6*30+1 = 181 divides 6^30+1 = 221073919720733357899777. %t A381257 Select[Range[0, 866], PowerMod[6, #, 6#+1]==6#&] (* _James C. McMahon_, Apr 02 2025 *) %o A381257 (PARI) isok(n) = my(m=6*n+1); Mod(6, m)^n==-1 %Y A381257 Cf. A224486, A222948, A230076, A381256, A381258. %K A381257 nonn %O A381257 1,3 %A A381257 _René-Louis Clerc_, Feb 18 2025