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 A309383 #14 Jul 28 2019 10:26:26 %S A309383 5,13,25,73,361,361,2521,2521,5041,5041,5041,5041,55441,55441,277201, %T A309383 3603601,10810801,10810801,10810801,21621601,21621601,367567201, %U A309383 367567201,367567201 %N A309383 a(n) is the smallest b > 1 such that when c is equal to any of the first n composites the congruence b^(c-1) == 1 (mod c) is satisfied, i.e., smallest b larger than 1 such that any member of the set of smallest n composites is a base-b Fermat pseudoprime. %e A309383 For n = 4: The four smallest composites are 4, 6, 8, 9 and for those four values of c the congruence b^(c-1) == 1 (mod c) is satisfied with b = 73. Since 73 is the smallest such value of b > 1, a(4) = 73. %o A309383 (PARI) composites(n) = my(v=[]); forcomposite(c=1, , v=concat(v, [c]); if(#v >= n, return(v))) %o A309383 a(n) = my(cp=composites(n)); for(b=2, oo, for(k=1, #cp, if(Mod(b, cp[k])^(cp[k]-1)!=1, break, if(k==#cp, return(b))))) %Y A309383 Cf. A242742, A256236. %K A309383 nonn,more %O A309383 1,1 %A A309383 _Felix Fröhlich_, Jul 27 2019