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 A177415 #16 Sep 08 2019 10:05:34 %S A177415 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 A177415 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 A177415 157,151,7,3,7,101,89,3,7,59,97,11,3,5,11,61,167,5,53,3,149,11,151 %N A177415 Smallest prime divisor of the n-th base-2 pseudoprime. %H A177415 Amiram Eldar, <a href="/A177415/b177415.txt">Table of n, a(n) for n = 1..10000</a> %F A177415 a(n) = A020639(A001567(n)). %t A177415 lpf[n_]:=FactorInteger[n][[1, 1]]; lpf /@ Select[Range[1, 10^4, 2], CompositeQ[#] && PowerMod[2, #-1, # ] == 1 &] (* _Amiram Eldar_, Sep 08 2019 *) %o A177415 (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 A177415 /* Print initial 80 terms as follows */ %o A177415 terms(80) \\ _Felix Fröhlich_, Sep 08 2019 %Y A177415 Cf. A001567, A020639. %K A177415 nonn %O A177415 1,1 %A A177415 _Juri-Stepan Gerasimov_, Dec 10 2010 %E A177415 Corrected by _D. S. McNeil_, Dec 11 2010