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A049011 Composite numbers k such that number of composite d with 3 < d < k, gcd(k, d) = 1, is pi(k). 0
27, 286, 370, 520, 550, 1332, 13530, 38220 (list; graph; refs; listen; history; text; internal format)
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

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Last modified May 16 12:46 EDT 2021. Contains 343947 sequences. (Running on oeis4.)