The main purpose of this Note is to provide a non-trivial bound on certain Kloosterman sums as considered in the recent paper of E. Fouvry [F], leading to a small improvement of his result on the Pell equation.
On Kloosterman sums with multiplicative coefficients
Kloosterman sums do not correlate with periodic functions
Sign changes of Kloosterman sums and exceptional characters
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