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A079446 Integers k such that phi(k)*(k-phi(k))+1 is prime. 3
2, 3, 4, 5, 7, 8, 9, 11, 13, 17, 18, 19, 20, 21, 23, 25, 27, 28, 29, 31, 32, 37, 41, 43, 47, 50, 52, 53, 55, 56, 57, 59, 61, 67, 68, 71, 72, 73, 77, 78, 79, 81, 83, 89, 92, 95, 97, 99, 101, 103, 104, 107, 108, 109, 110, 113, 115, 116, 127, 131, 137, 139, 145 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Includes all primes. - Robert Israel, Oct 25 2017

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

phi(8)=4, so 4*(8-4)+1=17.

MAPLE

filter:= proc(n) local p;

  p:= numtheory:-phi(n);

  isprime(p*(n-p)+1)

end proc:

select(filter, [$1..1000]); # Robert Israel, Oct 25 2017

MATHEMATICA

Select[Range[150], PrimeQ[EulerPhi[#]*(# - EulerPhi[#]) + 1] &] (* G. C. Greubel, Jan 18 2019 *)

PROG

(PARI) for (n=2, 100, if (isprime(eulerphi(n)*(n-eulerphi(n))+1), print1(n", ")))

CROSSREFS

Cf. A079445.

Sequence in context: A038701 A127072 A056781 * A322546 A283262 A115975

Adjacent sequences:  A079443 A079444 A079445 * A079447 A079448 A079449

KEYWORD

nonn

AUTHOR

Jon Perry, Jan 09 2003

EXTENSIONS

More terms from Michel Marcus, Mar 17 2014

STATUS

approved

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Last modified February 21 03:06 EST 2019. Contains 320364 sequences. (Running on oeis4.)