Short Kloosterman sums to powerful modulus

Resumo

An estimate of Kloosterman sums with very few terms is obtained. The modulus corresponding to this sum is a powerful number, i.e., the radical (the product of all prime factors) of the modulus is very small in comparison with the modulus itself on the logarithmic scale. The proof essentially uses methods of A.G. Postnikov and H. Iwaniec.