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%I #16 Sep 08 2019 10:05:34
%S 11,3,3,5,19,7,3,23,5,37,7,29,37,17,3,31,43,7,73,53,3,7,31,5,5,3,7,59,
%T 11,43,23,7,5,3,97,71,3,97,3,13,7,17,89,73,103,3,5,89,5,7,5,127,13,
%U 157,151,7,3,7,101,89,3,7,59,97,11,3,5,11,61,167,5,53,3,149,11,151
%N Smallest prime divisor of the n-th base-2 pseudoprime.
%H Amiram Eldar, <a href="/A177415/b177415.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = A020639(A001567(n)).
%t lpf[n_]:=FactorInteger[n][[1, 1]]; lpf /@ Select[Range[1, 10^4, 2], CompositeQ[#] && PowerMod[2, #-1, # ] == 1 &] (* _Amiram Eldar_, Sep 08 2019 *)
%o (PARI) terms(n) = my(i=0); forcomposite(c=1, , if(i>=n, break, if(Mod(2, c)^(c-1)==1, print1(factor(c)[1, 1], ", "); i++)))
%o /* Print initial 80 terms as follows */
%o terms(80) \\ _Felix Fröhlich_, Sep 08 2019
%Y Cf. A001567, A020639.
%K nonn
%O 1,1
%A _Juri-Stepan Gerasimov_, Dec 10 2010
%E Corrected by _D. S. McNeil_, Dec 11 2010
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