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A165767
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Numbers m such that 2^m-m is a semiprime.
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4
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6, 7, 15, 18, 25, 31, 33, 39, 42, 45, 49, 62, 73, 85, 93, 103, 119, 171, 187, 193, 199, 201, 269, 367, 379, 405, 413, 449, 459, 481, 489, 549, 577, 601, 631, 669, 787, 795, 1399
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refs;
listen;
history;
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internal format)
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OFFSET
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1,1
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COMMENTS
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The largest resp. smallest prime factor of 2^a(n)-a(n) is listed in A165768 resp. A165769.
1501, 1587, 1667, 2250, 3393, 5845, 9967, 16147 are terms of this sequence. - Chai Wah Wu, Oct 18 2019
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LINKS
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FORMULA
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a(n)=2k <=> 4^k/2-k is prime <=> A165768(n)=2.
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EXAMPLE
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199 is in this sequence because 2^199-199 = 17377902756647509 * 46235097144973199564251065756966919577339221 and these two factors are prime.
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MATHEMATICA
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PROG
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(PARI) for( i=1, 200, bigomega(2^i-i)==2 & print1(i", "))
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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