Given a polynomial $Q(x_1,\cdots, x_t)=\lambda_1 x_1^{ k_1}+\cdots +\lambda_t x_t^{ k_t}$, for every $c\in \mathbb{ Z}$ and $n\geq 2$, we study the number of solutions $N_J(Q;c,n)$ of the congruence equation $Q(x_1,\cdots, x_t)\equiv c\bmod{ n}$ in $(\mathbb{ Z}/n\mathbb{ Z})^t$ such that $x_i\in (\mathbb{ Z}/n\mathbb{ Z})^\times$ for $i\in J\subseteq I= \{ 1,\cdots, t\}$. We deduce formulas and an algorithm to study $N_J(Q; c,p^a)$ for $p$ any prime number and $a\geq 1$ any integer. As consequences of our main results, we completely solve: the counting problem of $Q(x_i)=\sum\limits_{ i\in I}\lambda_i x_i$ for any prime $p$ and any subset $J$ of $I$; the counting problem of $Q(x_i)=\sum\limits_{ i\in I}\lambda_i x^2_i$ in the case $t=2$ for any $p$ and $J$, and the case $t$ general for any $p$ and $J$ satisfying $\min\{ v_p(\lambda_i)\mid i\in I\}=\min\{ v_p(\lambda_i)\mid i\in J\}$; the counting problem of $Q(x_i)=\sum\limits_{ i\in I}\lambda_i x^k_i$ in the case $t=2$ for any $p\nmid k$ and any $J$, and in the case $t$ general for any $p\nmid k$ and $J$ satisfying $\min\{ v_p(\lambda_i)\mid i\in I\}=\min\{ v_p(\lambda_i)\mid i\in J\}$.
Categories: math.NT