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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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3
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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; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| A072281 with the primes removed; intersection of A066071 and A078893. - Ray Chandler (rayjchandler(AT)sbcglobal.net), May 26 2008
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EXAMPLE
| n=21,26,28,36,42 provide EulerPhi[n]=12 and the corresponding twins are {11,13}.
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MATHEMATICA
| 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
| Cf. A000010, A000040, A014574, A066071, A072281, A078893, A068017.
Sequence in context: A024884 A031009 A120184 * A054011 A114842 A169928
Adjacent sequences: A068016 A068017 A068018 * A068020 A068021 A068022
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KEYWORD
| nonn
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AUTHOR
| Labos E. (labos(AT)ana.sote.hu), Feb 08 2002
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