On Restricting No-Junta Boolean Function and Degree Lower Bounds by Polynomial Method

Let $mathcal{F}_{n}^*$ be the set of Boolean functions depending on all $n$ variables. We prove that for any $fin mathcal{F}_{n}^*$, $f|_{x_i=0}$ or $f|_{x_i=1}$ depends on the remaining $n-1$ variables, for some variable $x_i$. This existent result suggests a possible way to deal with general Boolean functions via its subfunctions of some restrictions. As an application, we consider the degree lower bound of representing polynomials over finite rings. Let $fin mathcal{F}_{n}^*$ and denote the exact representing degree over the ring $mathbb{Z}_m$ (with the integer $m>2$) as $d_m(f)$. Let $m=Pi_{i=1}^{r}p_i^{e_i}$, where $p_i$s are distinct primes, and $r$ and $e_i$s are positive integers. If $f$ is symmetric, then $mcdot d_{p_1^{e_1}}(f)... d_{p_r^{e_r}}(f) > n$. If $f$ is non-symmetric, by the second moment method we prove almost always $mcdot d_{p_1^{e_1}}(f)... d_{p_r^{e_r}}(f) > lg{n}-1$. In particular, as $m=pq$ where $p$ and $q$ are arbitrary distinct primes, we have $d_p(f)d_q(f)=Omega(n)$ for symmetric $f$ and $d_p(f)d_q(f)=Omega(lg{n}-1)$ almost always for non-symmetric $f$. Hence any $n$-variate symmetric Boolean function can have exact representing degree $o(sqrt{n})$ in at most one finite field, and for non-symmetric functions, with $o(sqrt{lg{n}})$-degree in at most one finite field.