A100410 Numbers m such that m is the phi(m)-th composite number (m = A002808(phi(m))).

6, 15, 21, 265, 295, 2681, 174757, 1302197, 237875719, 358334927, 26530938403, 26965177391, 127649964961, 409720883023

Offset

1,1

Comments

m is in the sequence iff m = phi(m) + pi(m) + 1.

a(15) > 2*10^12. - Giovanni Resta, Mar 31 2017

Links

Table of n, a(n) for n=1..14.

Example

15 is in the sequence because phi(15)=8 & 15=A002808(8).

Mathematica

Do[If[n==EulerPhi[n]+PrimePi[n]+1, Print[n]], {n, 75000000}]

Prog

(PARI) isok(n) = eulerphi(n)+primepi(n) == n - 1; \\ Michel Marcus, Apr 01 2017

Crossrefs

Cf. A100411, A002808, A037170.

Sequence in context: A362211 A072521 A130178 * A095032 A227610 A238905

Adjacent sequences: A100407 A100408 A100409 * A100411 A100412 A100413

Keyword

more,nonn

Author

Farideh Firoozbakht, Dec 01 2004

Extensions

a(9)-a(10) from Donovan Johnson, Aug 28 2012

a(11)-a(14) from Giovanni Resta, Mar 31 2017

Status

approved