A100410 - OEIS

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

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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

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

KEYWORD

more,nonn

AUTHOR

EXTENSIONS

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

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

STATUS

approved