For every positive integer denote by the sum of all its divisors, and by the number of positive integers less than and coprime with . Prove that there exists infinitely many such positive integer numbers for which .
Solution
Consider number , where is a prime number. Then . Let's prove that for infinitely many primes .
Let . Then
Notice that every , . Really, as it follows from the little Fermat theorem, , and as well, therefore , because is a prime number.
Hence and so . I.e. for sufficiently large .
Then
But as , therefore the number satisfies the condition for sufficiently large .
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