

A260476


Primes p such that p = 2*phi(sigma((p1)/2))+ 1.


1



3, 5, 17, 257, 65537, 285121, 1425601, 2380801, 100638721, 8778792961, 184354652161
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OFFSET

1,1


COMMENTS

The first 5 known Fermat primes from A019434 are in sequence.


LINKS

Table of n, a(n) for n=1..11.


EXAMPLE

17 = 2*phi(sigma((171)/2)+1 = 2*phi(15)+1 = 2*8+1, so 17 is in the sequence.


MATHEMATICA

Select[Prime@ Range@ 1000000, # == 2 EulerPhi[DivisorSigma[1, (#  1)/2]] + 1 &] (* Michael De Vlieger, Sep 25 2015 *)


PROG

(MAGMA) [n: n in [3..1000000]  IsPrime(n) and n eq 2 * EulerPhi(SumOfDivisors((n1) div 2)) + 1]
(PARI) forprime(p=3, 1e8, if((2*eulerphi(sigma((p1)/2)) + 1) == p, print1(p ", "))) \\ Altug Alkan, Sep 25 2015


CROSSREFS

Cf. A019434, A062401.
Sequence in context: A093179 A067387 A050922 * A070592 A254576 A232720
Adjacent sequences: A260473 A260474 A260475 * A260477 A260478 A260479


KEYWORD

nonn,more


AUTHOR

Jaroslav Krizek, Sep 24 2015


STATUS

approved



