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edited Apr 13, 2017 at 12:58

The formula is given in this question this question, while the answers thereto discuss the accuracy of the obvious approximation (that this is like $a \phi(n)/n.$)

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created Dec 2, 2016 at 10:33

Igor Rivin

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The formula is given in this question, while the answers thereto discuss the accuracy of the obvious approximation (that this is like $a \phi(n)/n.$)