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Ravi is an armchair futurist and an aspiring mad scientist. His mission is to create simplicity out of complexity and order out of chaos.

Friday, August 17, 2012

Schwartz-Zippel Lemma

The intuition behind this lemma seems rather straight-forward. An algebraic multi-variate equation of total degree $d$ can have no more than $d$ roots. In other words, a maximum of $d$ values (or elements from a field, if you will) can satisfy that algebraic equation (unless, of course, the equation is identical to zero, in which case any element satisfies the equation).

  • $x^2-3x+5$ has total degree 2. So it can have no more than 2 roots.
  • $x^3+x^2y^2+y^3$ has total degree 4. So it can have no more than 4 roots.

With the above intuition, it seems "obvious" that for any given set of elements (no matter how large or small), at most $d$ elements from that set can satisfy the given equation. So, the probability that a random element from this set will satisfy the equation is no more than $d/|S|$. And that is exactly the statement of this lemma.

Let $\mathbb{Q}(x_1,\cdots,x_n)\in\mathbb{F}[x_1,\cdots\,x_n]$ be a multi-variate polynomial of total degree $d$. Fix any finite set $S\subseteq\mathbb{F}$, and let $r_1,\cdots,r_n$ be chosen independently and uniformly at random from $S$. Then \[\mathbf{Pr}[\mathbb{Q}(r_1,\cdots,r_n)=0\hspace{1mm}|\hspace{1mm}\mathbb{Q}(x_1,\cdots,x_n)\not\equiv 0]\leq\frac{d}{|S|}\]

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