On the Density of Integer Points on the Generalised Markoff-Hurwitz and   Dwork Hypersurfaces
  We use bounds of mixed character sums modulo a prime $p$ to estimate the density of integer points on the hypersurface $$ f_1(x_1) + \ldots + f_n(x_n) =a x_1^{k_1} \ldots x_n^{k_n} $$ for some polynomials $f_i \in {\mathbb Z}[X]$, nonzero integer $a$ and positive integers $k_i$ $i=1, \ldots, n$. In the case of $$ f_1(X) = \ldots = f_n(X) = X^2 \quad \text{and}\quad k_1 = \ldots = k_n =1 $$ the above congruence is known as the Markoff-Hurwitz hypersurface, while for $$ f_1(X) = \ldots = f_n(X) = X^n\quad \text{and}\quad k_1 = \ldots = k_n =1 $$ it is known as the Dwork hypersurface. Our result is substantially stronger than those known for general hypersurfaces. 
Igor E. Shparlinski
math.NT