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 A339531 #19 Aug 18 2025 15:44:03
%S A339531 17,37,53,73,89,109,125,145,161,181,197,217,233,253,269,289,305,325,
%T A339531 341,361,377,397,413,433,449,469,485,505,521,541,557,577,593,613,629,
%U A339531 649,665,685,701,721,737,757,773,793,809,829,845,865,881,901,917,937,953
%N A339531 Numbers b > 1 such that the smallest two primes, i.e., 2 and 3 are base-b Wieferich primes.
%H A339531 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F A339531 a(n) = 4*A263941(n) + 1 for n>=2, a(n) = 4*floor((9*n)/2) + 1 for all n. - _Hugo Pfoertner_, Dec 08 2020
%F A339531 From _Chai Wah Wu_, Aug 18 2025: (Start)
%F A339531 a(n) = a(n-1) + a(n-2) - a(n-3) for n > 3.
%F A339531 G.f.: x*(-x^2 + 20*x + 17)/((x - 1)^2*(x + 1)). (End)
%t A339531 Select[Range[2, 10^3], Function[b, AllTrue[{2, 3}, PowerMod[b, (# - 1), #^2] == 1 &]]] (* _Michael De Vlieger_, Dec 10 2020 *)
%o A339531 (PARI) is(n) = forprime(p=1, 3, if(Mod(n, p^2)^(p-1)!=1, return(0))); 1
%Y A339531 Cf. A256236, A263941. Row 1 of A319059.
%Y A339531 Cf. smallest k primes are base-b Wieferich primes: A339532 (k=3), A339533 (k=4), A339534 (k=5), A339535 (k=6), A339536 (k=7), A339537 (k=8).
%K A339531 nonn
%O A339531 1,1
%A A339531 _Felix Fröhlich_, Dec 08 2020