login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A063999 Numbers k such that the number of primes <= k is phi(phi(k)). 1
2, 37, 55, 91, 95, 136, 178, 226, 507, 723, 1166, 1168, 1284, 1323, 2658, 3082, 7564, 8166, 11578, 12014, 12018, 12026, 15340, 40220, 50544, 55490, 56810, 69732, 89210, 100114, 100126, 111306, 131532, 226716, 226722, 232938, 519312, 683240, 849534 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..44

EXAMPLE

37 belongs to the sequence because number of primes <= 37 is equal to 12 (counting 2,3,5,7,11,13,17,19,23,29,31,37), while phi(37)=36 (counting 1,2,...,36) and phi(36)=12 (counting 1,5,7,11,13,17,19,23,25,29,31,35).

MAPLE

with(numtheory): p:=proc(n) if pi(n)=phi(phi(n)) then n else fi end: seq(p(n), n=1..900000); #  Emeric Deutsch, Feb 24 2005

PROG

(PARI) pi(n) = s=0; for(x=1, n, if(isprime(x), s++)); s; for(n=1, 10^6, if(pi(n)==eulerphi(eulerphi(n)), print(n)))

(PARI) { default(primelimit, 2500000); n=0; for (m=1, 10^9, if (primepi(m)==eulerphi(eulerphi(m)), write("b063999.txt", n++, " ", m); if (n==44, break)) ) } \\ Harry J. Smith, Sep 05 2009

CROSSREFS

Cf. A000720, A010554.

Sequence in context: A107180 A162577 A199980 * A062606 A282639 A099533

Adjacent sequences:  A063996 A063997 A063998 * A064000 A064001 A064002

KEYWORD

nonn

AUTHOR

Jason Earls, Sep 06 2001

EXTENSIONS

More terms from Emeric Deutsch, Feb 24 2005

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified July 4 16:24 EDT 2020. Contains 335448 sequences. (Running on oeis4.)