Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #4 May 26 2019 15:01:17
%S 10768,24599,27557,29957,33301,35397,45009,49085,55241,60661,63290,
%T 69389,89101,90607,93657,101305,103751,105657,117618,120473,123824,
%U 126449,129473,132857,133524,133948,138149,138896,143882,147717,150793,152985,159317,168920
%N Bases b where exactly nine primes p with p < b exist such that p is a base-b Wieferich prime.
%C Numbers n such that A255920(n) = 9.
%o (PARI) is(n) = my(i=0); forprime(p=1, n-1, if(Mod(n, p^2)^(p-1)==1, i++)); i==9
%Y Cf. A255920.
%Y Cf. bases b with exactly k base-b Wieferich primes less than b: A255921 (k=0), A255922 (k=1), A255923 (k=2), A255924 (k=3), A255925 (k=4), A325881 (k=5), A325882 (k=6), A325883 (k=7), A325884 (k=8), A325886 (k=10).
%K nonn
%O 1,1
%A _Felix Fröhlich_, May 26 2019