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 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

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Last modified September 23 07:26 EDT 2021. Contains 347609 sequences. (Running on oeis4.)