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 A257752 #15 Sep 16 2015 00:46:37 %S A257752 221,323,899,935,1105,1147,1271,1591,1595,1885,2093,2465,2821,4757, %T A257752 4807,4991,5609,5963,6497,7081,7843,9991,10373,10403,10961,11009, %U A257752 12319,13843,14111,16031,17155,17399,17653,17963,19043,19721,20701,24613,27331,28417,29341 %N A257752 Quasi-Carmichael numbers to exactly two bases. %H A257752 Tim Johannes Ohrtmann, <a href="/A257752/b257752.txt">Table of n, a(n) for n = 1..1487</a> %e A257752 a(1) = 221 because this is the first squarefree composite number n such that exactly two integers b except 0 exist such that for every prime factor p of n, p+b divides n+b (-11, -5): 221=13*17 and 2, 6 both divide 210 and 8, 12 both divide 216. %o A257752 (PARI) for(n=2, 1000000, if(!isprime(n), if(issquarefree(n), f=factor(n); k=0; for(b=-(f[1, 1]-1), n, c=0; for(i=1, #f[, 1], if((n+b)%(f[i, 1]+b)>0, c++)); if(c==0, if(!b==0, k++))); if(k==2, print1(n, ", "))))) %Y A257752 Cf. A257750 (every number of bases). %Y A257752 Cf. A257751, A257753, A257754, A257755, A257756, A257757, A258842 (1 and 3 to 8 bases). %Y A257752 Cf. A257758 (first occurrences). %K A257752 nonn %O A257752 1,1 %A A257752 _Tim Johannes Ohrtmann_, May 07 2015