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 A381010 #29 Apr 23 2025 16:58:24 %S A381010 1,7,511,713,11023,15553,43873,81079,95263,323593,628153,2275183, %T A381010 6520633,6955513,7947583,10817233,12627943,14223823,15346303,19852423, %U A381010 27923663,28529473,29360527,31019623,39041863,41007823,79015273,134217727,143998193,213444943,227018383 %N A381010 Positive integers k such that 2^(k+2) - 1 is divisible by k. %C A381010 7 is the only prime term. %p A381010 q:= k-> 0=(2&^(k+2)-1) mod k: %p A381010 select(q, [$1..1000000])[]; # _Alois P. Heinz_, Apr 10 2025 %o A381010 (Python) %o A381010 def in_sequence(n): %o A381010 return pow(2, n + 2, n) == 1 % n %o A381010 (PARI) isok(k) = Mod(2, k)^(k+2) == 1; \\ _Michel Marcus_, Apr 10 2025 %Y A381010 Cf. A000225, A055685, A069927, A187787. %K A381010 nonn %O A381010 1,2 %A A381010 _OisÃn Flynn-Connolly_, Apr 10 2025