A solution to erdos_418 was shown by Browkin and Schinzel [BrSc95] by
showing that any integer of the form $2^(k + 1)\cdot 509203$ is not of the
form $n - \phi(n)$.
[BrSc95] Browkin, J. and Schinzel, A., _On integers not of the form {$n-\phi(n)$}_. Colloq. Math. (1995), 55-58.
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