A049011 Composite numbers k such that number of composite d with 3 < d < k, gcd(k, d) = 1, is pi(k).

27, 286, 370, 520, 550, 1332, 13530, 38220

Offset

1,1

Comments

Composite numbers k such that phi(k) + omega(k) = 2*pi(k) + 1. - Jinyuan Wang, Sep 05 2020

Links

Table of n, a(n) for n=1..8.

Example

gcd(27,d)=1: d=4,8,10,14,16,20,22,25,26, pi(27)=9, so 27 is a term.

Prog

(PARI) isok(n) = {if (isprime(n) , return (0)); nb = 0; forcomposite (d=4, n-1, if (gcd(n, d) == 1, nb++); ); return (nb == primepi(n)); } \\ Michel Marcus, Jul 14 2013

Crossrefs

Cf. A036997.

Sequence in context: A059603 A224117 A251080 * A115246 A202126 A022687

Adjacent sequences: A049008 A049009 A049010 * A049012 A049013 A049014

Keyword

nonn,fini,full

Author

Naohiro Nomoto

Extensions

a(6)-a(8) from Michel Marcus, Jul 14 2013

Status

approved