OFFSET
1,1
COMMENTS
Inspired by a comment from Robert G. Wilson v.
All terms are composite.
Initial examples of forms of psi(k) - phi(k) where p, q, r, t are primes and a, b, c, d >= 1 as below:
If k = p^a, then psi(k) - phi(k) = 2*k/p.
If k = p^a*q^b, then psi(k) - phi(k) = 2*k*(p + q)/(p*q).
If k = p^a*q^b*r^c, then psi(k) - phi(k) = 2*k*(p*q + q*r + p*r + 1)/(p*q*r).
If k = p^a*q^b*r^c*t^d, then psi(k) - phi(k) = 2*k*(p*q*r + p*q*t + p*r*t + q*r*t + p + q + r + t)/(p*q*r*t).
EXAMPLE
6 is a term because psi(k) - phi(k) = 12 has no solution for any possible form of k.
MATHEMATICA
psi[n_] := If[n == 1, 1, n Times @@ (1 + 1/First /@ FactorInteger@ n)]; upto[n_] := Block[{d, T = 0 Range[n]}, Do[d = (psi[k] - EulerPhi[k])/2; If[d <= n, T[[d]] = 1], {k, 2, n^2}]; Flatten@ Position[T, 0]]; upto[387] (* Giovanni Resta, Sep 25 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Altug Alkan, Sep 24 2017
STATUS
approved