Heged\H{u}s's lemma is the following combinatorial statement regarding
polynomials over finite fields. Over a field $\mathbb{F}$ of characteristic $p
> 0$ and for $q$ a power of $p$, the lemma says that any multilinear polynomial
$P\in \mathbb{F}[x_1,\ldots,x_n]$ of degree less than $q$ that vanishes at all
points in $\{0,1\}^n$ of some fixed Hamming weight $k\in [q,n-q]$ must also
vanish at all points in $\{0,1\}^n$ of weight $k + q$. This lemma was used by
Heged\H{u}s (2009) to give a solution to \emph{Galvin's problem}, an extremal
problem about set systems; by Alon, Kumar and Volk (2018) to improve the
best-known multilinear circuit lower bounds; and by Hrube\v{s}, Ramamoorthy,
Rao and Yehudayoff (2019) to prove optimal lower bounds against depth-$2$
threshold circuits for computing some symmetric functions.
In this paper, we formulate a robust version of Heged\H{u}s's lemma.
Informally, this version says that if a polynomial of degree $o(q)$ vanishes at
most points of weight $k$, then it vanishes at many points of weight $k+q$. We
prove this lemma and give three different applications.