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Search: id:A068019
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| A068019 |
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Composite n such that both 1+EulerPhi[n] and -1+EulerPhi[n] are primes, i.e. EulerPhi[n] is middle term between twin primes (A014574). |
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4
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| 8, 9, 10, 12, 14, 18, 21, 26, 27, 28, 36, 38, 42, 49, 54, 62, 77, 86, 91, 93, 95, 98, 99, 111, 117, 122, 124, 133, 135, 146, 148, 152, 154, 171, 182, 186, 189, 190, 198, 206, 209, 216, 217, 218, 221, 222, 228, 234, 252, 266, 270, 278, 279, 287, 291, 297, 302
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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A072281 with the primes removed; intersection of A066071 and A078893. - Ray Chandler (rayjchandler(AT)sbcglobal.net), May 26 2008
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EXAMPLE
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n=21,26,28,36,42 provide EulerPhi[n]=12 and the corresponding twins are {11,13}.
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MATHEMATICA
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Do[s=-1+EulerPhi[n]; s1=1+EulerPhi[n]; If[PrimeQ[s]&&PrimeQ[s1]&&!PrimeQ[n], Print[n]], {n, 1, 2000}]
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CROSSREFS
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Cf. A000010, A000040, A014574, A066071, A072281, A078893, A068017.
Sequence in context: A024884 A031009 A120184 this_sequence A054011 A114842 A067683
Adjacent sequences: A068016 A068017 A068018 this_sequence A068020 A068021 A068022
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KEYWORD
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nonn
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AUTHOR
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Labos E. (labos(AT)ana.sote.hu), Feb 08 2002
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