Randomized Algorithms (Spring 2010)/Fingerprinting

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Fingerprinting

Evaluating at random points

Example: Checking matrix multiplication

Consider the following problem:

  • Given as the input three [math]\displaystyle{ n\times n }[/math] matrices [math]\displaystyle{ A,B }[/math] and [math]\displaystyle{ C }[/math],
  • check whether [math]\displaystyle{ C=AB }[/math].
Algorithm (Freivalds)
  • pick a vector [math]\displaystyle{ r \in\{0, 1\}^n }[/math] uniformly at random;
  • if [math]\displaystyle{ A(Br) = Cr }[/math] then return "yes" else return "no";

If [math]\displaystyle{ AB=C }[/math] then [math]\displaystyle{ A(Br) = Cr }[/math] for any [math]\displaystyle{ r \in\{0, 1\}^n }[/math], thus the algorithm always returns "yes".

Lemma
If [math]\displaystyle{ AB\neq C }[/math] then for a uniformly random [math]\displaystyle{ r \in\{0, 1\}^n }[/math],
[math]\displaystyle{ \Pr[A(Br) = Cr]\le \frac{1}{2} }[/math].

Example: Checking polynomial identities

Consider the following problem:

  • Given as the input two multivariate polynomials [math]\displaystyle{ P_1(x_1,\ldots,x_n) }[/math] and [math]\displaystyle{ P_2(x_1,\ldots,x_n) }[/math],
  • check whether [math]\displaystyle{ P_1\equiv P_2 }[/math].
Algorithm (Schwartz-Zippel)
  • pick [math]\displaystyle{ r_1, \ldots , r_n }[/math] independently and uniformly at random from a set [math]\displaystyle{ S }[/math];
  • if [math]\displaystyle{ P_1(r_1, \ldots , r_n) = P_2(r_1, \ldots , r_n) }[/math] then return “yes” else return “no”;
Theorem (Schwartz-Zippel)
Let [math]\displaystyle{ Q(x_1,\ldots,x_n) }[/math] be a multivariate polynomial of degree [math]\displaystyle{ d }[/math] defined over a field [math]\displaystyle{ \mathbb{F} }[/math]. Fix any finite set [math]\displaystyle{ S\subset\mathbb{F} }[/math], and let [math]\displaystyle{ r_1,\ldots,r_n }[/math] be chosen independently and uniformly at random from [math]\displaystyle{ S }[/math]. Then
[math]\displaystyle{ \Pr[Q(r_1,\ldots,r_n)=0\mid Q\not\equiv 0]\le\frac{d}{|S|}. }[/math]

Evaluating over a random field

Example: Identity checking

Example: Randomized pattern matching

Universal hashing

Example: checking distinctness

Probabilistic Checkable Proofs (PCPs)