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A079446
Integers k such that phi(k)*(k-phi(k))+1 is prime.
2
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
OFFSET
1,1
COMMENTS
Includes all primes. - Robert Israel, Oct 25 2017
LINKS
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: A127072 A341089 A056781 * A322546 A283262 A334393
KEYWORD
nonn
AUTHOR
Jon Perry, Jan 09 2003
EXTENSIONS
More terms from Michel Marcus, Mar 17 2014
STATUS
approved