

A100411


Numbers n such that n is the phi(n)th nonprime number.


2



1, 4, 8, 27, 35, 215, 235, 2611, 65113, 174647, 1301317, 1302509, 5021647, 10885081, 36319939, 70112131, 70113559, 325575773, 514258883, 742327529, 1069238453, 18721771909, 27788563039, 113350570453, 835005036247
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OFFSET

1,2


COMMENTS

n is in the sequence iff n is't prime and n=phi(n)+pi(n). This sequence is a subsequence of A037170 and 2 is the only term of A037170 which is not in this sequence because 2 is the only prime p such that p=phi(p)+pi(p).
There is no further term up to 10^9.  Farideh Firoozbakht, Jul 26 2009
a(24) > 10^11.  Donovan Johnson, Feb 05 2010
a(26) > 2.3*10^12.  Giovanni Resta, Mar 28 2017


LINKS

Table of n, a(n) for n=1..25.


EXAMPLE

27 is in the sequence because phi(27)=18 and 27 is the 18th nonprime number, the first 17 nonprime numbers are 1,4,6,8,9,10,12,14,15,16,18,20,21,22,24,25 & 26.


MATHEMATICA

Select[Range[10^5], # == PrimePi[#] + EulerPhi[#] && ! PrimeQ[#] &] (* Giovanni Resta, Mar 28 2017 *)


CROSSREFS

Cf. A037170, A100410, A018252.
Sequence in context: A012952 A272588 A272228 * A065405 A279627 A026085
Adjacent sequences: A100408 A100409 A100410 * A100412 A100413 A100414


KEYWORD

more,nonn


AUTHOR

Farideh Firoozbakht, Dec 01 2004


EXTENSIONS

a(19)a(20) from Farideh Firoozbakht, Jul 26 2009
a(21)a(23) from Donovan Johnson, Feb 05 2010
a(24)a(25) from Giovanni Resta, Mar 28 2017


STATUS

approved



