On the number of zeros to the equation $f(x_1)+...+f(x_n)=a$ over finite fields

Let $p$ be a prime, $k$ a positive integer and let $mathbb{F}_q$ be the finite field of $q=p^k$ elements. Let $f(x)$ be a polynomial over $mathbb F_q$ and $ainmathbb F_q$. We denote by $N_{s}(f,a)$ the number of zeros of $f(x_1)+cdots+f(x_s)=a$. In this paper, we show that $$sum_{s=1}^{infty}N_{s}(f,0)x^s=frac{x}{1-qx} -frac{x { M_f^{prime}}(x)}{qM_f(x)},$$ where $$M_f(x):=prod_{minmathbb F_q^{ast}atop{S_{f, m} e 0}}Big(x-frac{1}{S_{f,m}}Big)$$ with $S_{f, m}:=sum_{xin mathbb F_q}zeta_p^{{rm Tr}(mf(x))}$, $zeta_p$ being the $p$-th primitive unit root and ${rm Tr}$ being the trace map from $mathbb F_q$ to $mathbb F_p$. This extends Richmans theorem which treats the case of $f(x)$ being a monomial. Moreover, we show that the generating series $sum_{s=1}^{infty}N_{s}(f,a)x^s$ is a rational function in $x$ and also present its explicit expression in terms of the first $2d+1$ initial values $N_{1}(f,a), ..., N_{2d+1}(f,a)$, where $d$ is a positive integer no more than $q-1$. From this result, the theorems of Chowla-Cowles-Cowles and of Myerson can be derived.