A048867 Numbers for which reduced residue system contains fewer primes than nonprimes.

1, 2, 11, 13, 17, 19, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 94, 95, 97, 99, 101, 103, 106, 107, 109, 111, 113, 115, 117, 118, 119, 121, 122, 123, 124, 125, 127, 128

Offset

1,2

Comments

Integers n such that 2*A000720(n) - 2*A001221(n) < A000010(n).

Links

G. C. Greubel, Table of n, a(n) for n = 1..1000

Example

Large primes belong to this set. Composites like: n=35 has 2 prime divisors, Phi(35)=24 Pi(35)=11; In reduced residue system 9 primes and 15 nonprimes occur.

Mathematica

Select[Range[500], 2*PrimePi[#] - 2*PrimeNu[#] < EulerPhi[#] &] (* G. C. Greubel, May 12 2017 *)

Prog

(PARI) isok(n) = 2*(primepi(n) - omega(n)) < eulerphi(n); \\ Michel Marcus, May 13 2017

Crossrefs

Cf. A000010, A000720, A001221.

Sequence in context: A286153 A068972 A116437 * A113626 A154539 A137238

Adjacent sequences: A048864 A048865 A048866 * A048868 A048869 A048870

Keyword

nonn

Author

Labos Elemer

Status

approved