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A072281
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Numbers n such that phi(n) + 1 and phi(n) - 1 are twin primes.
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2
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5, 7, 8, 9, 10, 12, 13, 14, 18, 19, 21, 26, 27, 28, 31, 36, 38, 42, 43, 49, 54, 61, 62, 73, 77, 86, 91, 93, 95, 98, 99, 103, 109, 111, 117, 122, 124, 133, 135, 139, 146, 148, 151, 152, 154, 171, 181, 182, 186, 189, 190, 193, 198, 199, 206, 209, 216, 217, 218, 221, 222
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Phi(n) is middle term between twin primes (A014574). Union of A006512 and A068019; intersection of A039698 and A078892. - Chandler
The positions of isolated nonprimes in A000010. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Nov 10 2009]
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FORMULA
| A000010(a(n))=isolated nonprime. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Nov 18 2009]
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EXAMPLE
| phi(14) + 1 = 7 and phi(14) - 1 = 5, so 14 is a term of the sequence.
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MATHEMATICA
| Select[Range[10^3], PrimeQ[EulerPhi[ # ] + 1] && PrimeQ[EulerPhi[ # ] - 1] &]
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CROSSREFS
| Cf. A000010, A000040, A006512, A014574, A039698, A068019, A078892.
Sequence in context: A038609 A078892 A164374 * A111339 A171097 A166460
Adjacent sequences: A072278 A072279 A072280 * A072282 A072283 A072284
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KEYWORD
| easy,nonn
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AUTHOR
| Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Jul 10 2002
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EXTENSIONS
| Extended by Ray Chandler (rayjchandler(AT)sbcglobal.net), May 26 2008
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