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 A217841 #32 Feb 16 2025 08:33:18 %S A217841 91,703,1891,2701,7381,8911,10585,12403,16471,18721,29161,38503,41041, %T A217841 49141,79003,88831,93961,104653,115921,146611,188191,218791,226801, %U A217841 269011,286903,314821,334153,364231,385003,497503,534061,597871,665281,721801,736291,765703,873181,954271,1056331,1237951 %N A217841 Fermat pseudoprimes n to base 3 for which sqrt(8*n + 1) is an integer. %H A217841 Charles R Greathouse IV, <a href="/A217841/b217841.txt">Table of n, a(n) for n = 1..10000</a> %H A217841 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FermatPseudoprime.html">Fermat Pseudoprime</a> %o A217841 (PARI) list(lim)=my(v=List(),n); lim\=1; forstep(k=27,sqrtint(8*lim+1),2, n=k^2>>3; if(Mod(3,n)^(n-1)==1, listput(v,n))); Vec(v) \\ _Charles R Greathouse IV_, Jun 30 2017 %Y A217841 Cf. A005935, A210461 (subsequence). %K A217841 nonn %O A217841 1,1 %A A217841 _Marius Coman_, Oct 12 2012 %E A217841 a(15)-a(18) and a(35) from _Charles R Greathouse IV_, Jun 30 2017