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 A375794 #18 Sep 20 2024 06:41:04
%S A375794 5,13,37,61,101,109,157,181,421,541,661,757,821,1093,1621,1861,2029,
%T A375794 2053,2269,2341,2437,2701,2917,3277,3301,3613,4621,4789,4861,5461,
%U A375794 5501,5581,6301,6661,7309,8101,8269,8581,8821,8893,9829,9901,10141,10261,10501,10837,11701,12101,12301
%N A375794 Numbers k such that 2^k == 2 (mod ((k - 1)*k/2)) and not 2^k == 2 (mod ((k - 1)*k)).
%C A375794 a(22) = 2701 is the first composite term of the sequence.
%t A375794 Select[Range[2,12400], PowerMod[2,#,(#-1)#/2]==2 && !PowerMod[2,#,(#-1)#]==2 &] (* _Stefano Spezia_, Sep 19 2024 *)
%o A375794 (Magma) [k: k in [2..13333] | Modexp(2, k, (k^2-k) div 2) eq 2 and not Modexp(2, k, k^2-k) eq 2];
%o A375794 (PARI) isok(k)={k > 1 && Mod(2, (k-1)*k)^k == 2 + (k-1)*k/2} \\ _Andrew Howroyd_, Aug 29 2024
%Y A375794 Cf. A069051, A375792.
%K A375794 nonn
%O A375794 1,1
%A A375794 _Juri-Stepan Gerasimov_, Aug 29 2024