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 A224486 #25 Oct 13 2020 14:35:57 %S A224486 1,2,5,6,9,14,18,21,26,29,30,33,41,50,53,54,65,69,74,78,81,86,89,90, %T A224486 98,105,113,114,125,134,138,141,146,153,158,165,173,174,186,189,194, %U A224486 198,209,210,221,230,233,245,249,254,261,270,273,278,281,285,293 %N A224486 Numbers k such that 2*k+1 divides 2^k+1. %C A224486 The numbers are called Curzon numbers by Tattersall (p. 85, exercise 43). %C A224486 Sequence 2*a(n)+1 apparently is A175865 (certainly it is not A003629). - _Joerg Arndt_, Apr 07 2013 %D A224486 James J. Tattersall, Elementary Number Theory in Nine Chapters, Second Edition, Cambridge University Press, 2005, p. 85. %H A224486 Amiram Eldar, <a href="/A224486/b224486.txt">Table of n, a(n) for n = 1..10000</a> %H A224486 Giovanni Resta, <a href="http://www.numbersaplenty.com/set/Curzon_number/">Curzon numbers</a>, Numbers Aplenty. %e A224486 5 is in the list since 2*5 + 1 = 11 divides 2^5 + 1 = 33. %t A224486 Select[Range[300], PowerMod[2, #, 2 # + 1] == 2 # &] (* _Amiram Eldar_, Oct 13 2020 *) %o A224486 (PARI) for(n=0, 10^3, my(m=2*n+1); if( Mod(2,m)^n==Mod(-1,m), print1(n, ", ") ) ); \\ _Joerg Arndt_, Apr 08 2013 %Y A224486 Cf. A000051, A175865, A224499. %K A224486 nonn %O A224486 1,2 %A A224486 _Jayanta Basu_, Apr 07 2013