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A067192
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Composite c such that sigma(c)==2 (mod phi(c)).
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3
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10, 20, 52, 232, 976, 1332, 65152, 261376, 4191232, 67096576, 274877120512, 4398043365376
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OFFSET
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1,1
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COMMENTS
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a(12) > 10^12. 4398043365376, 70368731594752 and 72057593635274752 are also terms. - Donovan Johnson, Feb 29 2012
a(13) > 10^13. If 2^k-3 is prime (A050414), then 2^(k-2)*(2^k-3) is a term. Up to 10^13 the only term not of this form is 1332. - Giovanni Resta, Mar 29 2020
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LINKS
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PROG
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(PARI) isok(c) = !isprime(c) && ((sigma(c) % eulerphi(c)) == 2); \\ Michel Marcus, Feb 17 2021
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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