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A336663
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2-Carmichael numbers: composite numbers n such that A^{n*(n-1)*(n+1)} = I for every matrix A from the group GL(2,Z/nZ).
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1
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4, 8, 9, 15, 16, 24, 25, 27, 32, 40, 45, 48, 49, 55, 63, 64, 72, 75, 80, 81, 96, 99, 104, 105, 112, 120, 121, 125, 128, 135, 144, 160, 165, 169, 171, 175, 176, 192, 195, 200, 216, 224, 225, 231, 240, 243, 256, 264, 273, 275, 288, 289, 320, 336, 343, 351, 360
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OFFSET
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1,1
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COMMENTS
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Theorem (an analogue of Korselt's criterion).
For a composite number n the following statements are equivalent:
(1) n is a 2-Carmichael number,
(2) for any prime divisor p of n, (p-1)*(p+1) | n*(n-1)*(n+1).
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LINKS
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MATHEMATICA
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twoCarmQ[n_] := CompositeQ[n] && AllTrue[FactorInteger[n][[;; , 1]], Divisible[(n - 1)*n*(n + 1), #^2 - 1] &]; Select[Range[360], twoCarmQ] (* Amiram Eldar, Dec 29 2020 *)
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PROG
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(PARI) is(m) = {my(f=factor(m)[, 1], t=m*(m^2-1)); !isprime(m+(m<2)) && !sum(i=1, #f, t%(f[i]^2-1)); } \\ Jinyuan Wang, Jul 29 2020
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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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