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 A271656 Odd primes p such that phi(p+1) = phi(phi(p-2)+1). 6
 5, 7, 17, 71, 257, 487, 761, 1151, 4327, 18287, 52721, 65537, 150767, 221471, 446597, 830861, 1002467, 2202787, 4978367, 6088337, 7534511, 9887651, 14278391, 21453101, 25032437, 31176863, 47290697, 51182837, 54264257, 80228717, 83623937, 138307877 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The first 4 known Fermat primes > 3 from A019434 are in the sequence. LINKS Michel Marcus, Table of n, a(n) for n = 1..70 EXAMPLE 257 is a term because phi(257+1) = phi(258) = 84 = phi(phi(257-2)+1) = phi(phi(255)+1) = phi(128+1) = phi(129). MATHEMATICA Select[Prime@ Range[10^6], EulerPhi[# + 1] == EulerPhi[EulerPhi[# - 2] + 1] &] (* Michael De Vlieger, Apr 11 2016 *) PROG (Magma) [n: n in [2..10^8] | IsPrime(n) and EulerPhi(n+1) eq EulerPhi(EulerPhi(n-2)+1)] (PARI) is(n) = eulerphi(n+1)==eulerphi(eulerphi(n-2)+1) forprime(p=3, , if(is(p), print1(p, ", "))) \\ Felix Fröhlich, Apr 23 2016 CROSSREFS Cf. A019434, A271655, A271657, A271658, A271659, A271660. Sequence in context: A123195 A248480 A243844 * A145582 A034762 A138919 Adjacent sequences: A271653 A271654 A271655 * A271657 A271658 A271659 KEYWORD nonn AUTHOR Jaroslav Krizek, Apr 11 2016 EXTENSIONS Definition adjusted and more terms added by Felix Fröhlich, Apr 23 2016 STATUS approved

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Last modified April 16 04:17 EDT 2024. Contains 371696 sequences. (Running on oeis4.)