

A110073


Numbers n such that sigma(n)=2nphi(phi(n)).


2



1, 2, 4, 10, 76, 410, 890, 1370, 2330, 5690, 8090, 8570, 10490, 10970, 11930, 14330, 19130, 21530, 27770, 32090, 34490, 35930, 38330, 39290, 40730, 44570, 47930, 49370, 52730, 54170, 60890, 64730, 65690, 68570, 74330, 75290, 75770, 78170
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OFFSET

1,2


COMMENTS

If p is an odd prime and 8p+1 is prime then n=10(8p+1) is in the sequence because 2nphi(phi(n))=20(8p+1)16(p1)=144p+3618*(8p+2)=sigma(n).
Conjecture: Each term which is greater than 76 of this sequence is of the form 80p+10 where both p and 8p+1 are primes.


LINKS



EXAMPLE

76 is in the sequence because sigma(76)=2*76phi(phi(76)).


MATHEMATICA

Do[If[DivisorSigma[1, m] == 2m  EulerPhi[EulerPhi[m]], Print[m]], {m, 100000}]


PROG



CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



