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A257643
Carmichael numbers k such that k-1 is squarefree.
2
139952671, 74689102411, 121254376891, 187054437571, 231440115271, 236359158267, 303008129971, 306252926071, 380574791611, 426951670531, 556303918171, 639109148371, 660950414671, 1101375141511, 1483826843731, 1487491483171, 1861175569891, 2794268624071
OFFSET
1,1
COMMENTS
If k is a Carmichael number with k-1 squarefree, then gcd(phi(k),k-1) = lambda(k), i.e., Carmichael lambda function A002322.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..5287 (terms below 10^22, calculated using data from Claude Goutier; terms 1..164 from Robert Israel, terms 165..1037 from Charles R Greathouse IV)
PROG
(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;
is(n) = n%2 && !isprime(n) && t(n) && n>1;
isok(n) = is(n) && issquarefree(n-1); \\ Altug Alkan, Nov 06 2015
(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)
is(n) = n%2 && !isprime(n) && t(n) && n>1 \\ Charles R Greathouse IV, Nov 09 2015
CROSSREFS
Subsequence of A185321.
Sequence in context: A109093 A217002 A036744 * A262532 A075130 A202280
KEYWORD
nonn
AUTHOR
Thomas Ordowski, Nov 05 2015
STATUS
approved