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 A380831 #5 Feb 05 2025 09:02:50 %S A380831 1023,1638,14670,21399,24570,40290,44178,45375,52326,98046,128499, %T A380831 135975,157410,229494,244998,257223,370875,400302,419430,436590, %U A380831 458163,502326,625974,686826,754854,839270,905786,993510,1102983,1134546,1142226,1152083,1193898,1373238,1374011 %N A380831 Numbers k such that k^(k + 1) == k + 1 (mod 2*k + 1) while 2*k+1 is not prime. %o A380831 (PARI) isok(k) = (!isprime(2*k+1)) && (Mod(k, 2*k+1)^(k+1) == k+1); %Y A380831 Intersection of A374913 and A047845. %K A380831 nonn %O A380831 1,1 %A A380831 _Michel Marcus_, Feb 05 2025