OFFSET
1,1
COMMENTS
EXAMPLE
17 = 2*phi(sigma((17-1)/2) + 1 = 2*phi(15) + 1 = 2*8 + 1, so 17 is in the sequence.
MATHEMATICA
Select[Range[10000], # == 2*EulerPhi[DivisorSigma[1, (# - 1)/2] ] + 1 &] (* G. C. Greubel, Dec 22 2015 *)
PROG
(Magma) [n: n in [3..10^7] | n eq 2*EulerPhi(SumOfDivisors((n-1) div 2)) + 1]
(Perl) use ntheory ":all"; for (1..1e7) { say if 2*euler_phi(divisor_sum(($_-1)>>1))+1 == $_ } # Dana Jacobsen, Dec 27 2015
(PARI) is(n)=n%2 && n>2 && 2*eulerphi(sigma((n-1)/2)) + 1 == n \\ Charles R Greathouse IV, Apr 25 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jaroslav Krizek, Dec 22 2015
EXTENSIONS
More terms from Dana Jacobsen, Dec 27 2015
STATUS
approved