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A263403
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Carmichael numbers k such that the odd part of k-1 is squarefree.
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1
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561, 1105, 2465, 2821, 62745, 75361, 278545, 530881, 3224065, 3581761, 4335241, 6049681, 7519441, 8355841, 9582145, 9890881, 10837321, 13696033, 17098369, 19384289, 22665505, 23382529, 26932081, 34657141, 36121345, 37167361, 40280065, 41471521, 43286881
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OFFSET
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1,1
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COMMENTS
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Are there Carmichael numbers k such that the odd part of k-1 is a Carmichael number?
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LINKS
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EXAMPLE
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a(1) = 561 because 561 is the first Carmichael number and the odd part of 560 is 35, which is squarefree.
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MATHEMATICA
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lim = 10^7; f[n_] := NestWhile[#/2 &, n, EvenQ]; t = Cases[Range[1, lim, 2], n_ /; Mod[n, CarmichaelLambda@ n] == 1 && ! PrimeQ@ n]; Select[t, SquareFreeQ@ f[# - 1] &] (* Michael De Vlieger, Oct 19 2015, after Artur Jasinski at A002997 *)
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PROG
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(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;
isC(n)=n%2 && !isprime(n) && t(n) && n>1;
is(n)=isC(n) && issquarefree((n-1) >> valuation((n-1), 2));
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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