Geodesic
Rational trigonometric sums along a curve
Zapiski Nauchnykh Seminarov POMI, Automorphic functions and number theory. Part II, Tome 134 (1984), pp. 232-251 
S. A. Stepanov. Rational trigonometric sums along a curve. Zapiski Nauchnykh Seminarov POMI, Automorphic functions and number theory. Part II, Tome 134 (1984), pp. 232-251. http://geodesic.mathdoc.fr/item/ZNSL_1984_134_a12/
@article{ZNSL_1984_134_a12,
     author = {S. A. Stepanov},
     title = {Rational trigonometric sums along a~curve},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {232--251},
     year = {1984},
     volume = {134},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1984_134_a12/}
}
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Voir la notice du chapitre de livre provenant de la source Math-Net.Ru

Under certain assumptions on the polynomials $f(x, y)$ and $g(x, y)$ the following estimate $$ \left|\sum_{\substack{x,y=1\\ f(x,y)\equiv0\pmod q}}e^\frac{2\pi ig(x, y)}q\right|\ll q^{1-\frac1{N+1}+\varepsilon},\quad\varepsilon>0 $$ is proved. There $N$ is the maximum over all $p|q$ of the intersection index of the curves $f(x, y)\equiv0\pmod p$ and $\frac{\partial f}{\partial y}\frac{\partial g}{\partial x}-\frac{\partial g}{\partial y}\frac{\partial f}{\partial x}\equiv0\pmod p$ in the finite field of $p$ elements.