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 A257643 #60 Apr 21 2024 09:59:19 %S A257643 139952671,74689102411,121254376891,187054437571,231440115271, %T A257643 236359158267,303008129971,306252926071,380574791611,426951670531, %U A257643 556303918171,639109148371,660950414671,1101375141511,1483826843731,1487491483171,1861175569891,2794268624071 %N A257643 Carmichael numbers k such that k-1 is squarefree. %C A257643 If k is a Carmichael number with k-1 squarefree, then gcd(phi(k),k-1) = lambda(k), i.e., Carmichael lambda function A002322. %H A257643 Amiram Eldar, <a href="/A257643/b257643.txt">Table of n, a(n) for n = 1..5287</a> (terms below 10^22, calculated using data from Claude Goutier; terms 1..164 from Robert Israel, terms 165..1037 from Charles R Greathouse IV) %H A257643 Claude Goutier, <a href="http://www-labs.iro.umontreal.ca/~goutier/OEIS/A055553/">Compressed text file carm10e22.gz containing all the Carmichael numbers up to 10^22</a>. %o A257643 (PARI) t(n) = my(f=factor(n)); for(i=1, #f[, 1], if(f[i, 2]>1||(n-1)%(f[i, 1]-1), return(0))); 1; %o A257643 is(n) = n%2 && !isprime(n) && t(n) && n>1; %o A257643 isok(n) = is(n) && issquarefree(n-1); \\ _Altug Alkan_, Nov 06 2015 %o A257643 (PARI) is(n) = my(f=factor(n)); for(i=1, #f~, if(f[i,1]%4<3 || f[i, 2]>1 || (n-1)%(f[i, 1]-1), return(0))); !isprime(n) && issquarefree(n-1) %o A257643 is(n) = n%2 && !isprime(n) && t(n) && n>1 \\ _Charles R Greathouse IV_, Nov 09 2015 %Y A257643 Subsequence of A185321. %Y A257643 Cf. A002997, A264012, A007674. %K A257643 nonn %O A257643 1,1 %A A257643 _Thomas Ordowski_, Nov 05 2015