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A072281 Numbers n such that phi(n) + 1 and phi(n) - 1 are twin primes. 9
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; text; internal format)
OFFSET

1,1

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, Nov 10 2009]

LINKS

Table of n, a(n) for n=1..61.

FORMULA

A000010(a(n))=isolated nonprime. [From Juri-Stepan Gerasimov, Nov 18 2009]

EXAMPLE

phi(14) + 1 = 7 and phi(14) - 1 = 5, so 14 is a term of the sequence.

MATHEMATICA

Select[Range[10^3], PrimeQ[EulerPhi[ # ] + 1] && PrimeQ[EulerPhi[ # ] - 1] &]

Select[Range[300], And@@PrimeQ[EulerPhi[#]+{1, -1}]&] (* Harvey P. Dale, Apr 07 2012 *)

CROSSREFS

Cf. A000010, A000040, A006512, A014574, A039698, A068019, A078892.

Sequence in context: A038609 A078892 A164374 * A242273 A261917 A111339

Adjacent sequences:  A072278 A072279 A072280 * A072282 A072283 A072284

KEYWORD

easy,nonn

AUTHOR

Joseph L. Pe, Jul 10 2002

EXTENSIONS

Extended by Ray Chandler, May 26 2008

STATUS

approved

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Last modified October 17 22:48 EDT 2018. Contains 316297 sequences. (Running on oeis4.)