Randomized Algorithms (Spring 2010)/Complexity classes and lower bounds

Computational Models

Upper bounds, lower bounds

Bounds are just inequalities. In Computer Science, when talking about upper or lower bounds, people really mean the upper or lower bounds of complexity.

Complexity is measured by the resource costed by the computation. Our most precious resource is time (life is short!). Besides time complexity, there are other measures of complexity we may care about, including:

• space;
• communication;
• number of random bits;
• number of queries to the input;
• amount of information provided by an oracle.

There are two fundamental ways of measuring complexity

Complexity of algorithms

For an algorithm [math]\displaystyle{ A }[/math], its complexity represents its performance. Let [math]\displaystyle{ T(A,x) }[/math] be the running time of the algorithm [math]\displaystyle{ A }[/math] on input [math]\displaystyle{ x }[/math]. The worst case time complexity is given by [math]\displaystyle{ T(A)=\max_{x}T(A,x) }[/math], where the maximum is taken of the domain of the inputs with the fixed length [math]\displaystyle{ n }[/math], and [math]\displaystyle{ T(A) }[/math] is written as a function of [math]\displaystyle{ n }[/math].

Complexity of problems

A computational problem can be formalized as a mapping from inputs to outputs. For a problem [math]\displaystyle{ f }[/math], its time complexity [math]\displaystyle{ T(f) }[/math] is defined as the [math]\displaystyle{ T(A) }[/math] for the best possible algorithm [math]\displaystyle{ A }[/math] that computes [math]\displaystyle{ f }[/math]. That is [math]\displaystyle{ T(f)=\min\{T(A)\mid A\mbox{ computes }f\} }[/math]. Again, [math]\displaystyle{ T(f) }[/math] is represented as a function of the length of the input [math]\displaystyle{ n }[/math].

The complexity of an algorithm tells how good a solution is, yet the complexity of a problem tells how hard a problem is. While the former is what we care mostly about in practice, the later is more about the fundamental truths of computation.

In Theoretical Computer Science, when talking about upper or lower bounds, people usually refer to the bounds of the complexity of a problem, rather than that of an algorithm.

Specifically, an upper bound (of the time complexity [math]\displaystyle{ T(f) }[/math] of the problem [math]\displaystyle{ f }[/math]) is an inequality in the form [math]\displaystyle{ T(f)\le t }[/math], expanding which, is

"there exists an algorithm [math]\displaystyle{ A }[/math] that computes [math]\displaystyle{ f }[/math], such that [math]\displaystyle{ T(A)\le t }[/math]".

And a lower bound [math]\displaystyle{ T(f)\ge t }[/math] means

"for any algorithm [math]\displaystyle{ A }[/math] that computes [math]\displaystyle{ f }[/math], its time complexity is at least [math]\displaystyle{ T(A)\ge t }[/math]", i.e. the necessary price that any algorithm that solves the problem has to pay.

Sometimes, the two terms "upper bound" and "lower bound" are abused so that an algorithm is called an upper bound and any bad news (impossibility results) can be called a lower bound.

Today's lecture is devoted to lower bounds, i.e. the necessary prices which have to paid by any algorithms which solve the given problems. Speaking of necessary prices, we have to be specific about the model, the mathematical formulations of problems and algorithms.