I am reading about square free values of quadratic polynomials:
Consider following conjecture:[
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Now consider the following expression
Number of elements in the set { ${n\leq X : D| f(n) } $ }
Author writes that he is decomposing the above expression and writes it equal to
$\sum_{C mod D, f(C) =0 mod D}$ Number of elements in the set { $n\leq X : n = C mod D$}.
But I am unable to see how it will be equal to the 2nd expression.
Can you please help with that.