## Complexity classes

November 02, 2017

The following notes are meant to act as a reminder for the most popular problem classes.

We indicate a generic problem with \(X\); it can be solved through different algorithms, we indicate with \(f\) a generic algorithm solving \(X\).

### Complexity class

We can classify \(f\) with respect to the following bounds:

- \(f\in \Omega(t(n)) \iff\) the cost (in terms of time or memory usage) of executing \(f\) increase faster than \(t(n)\)
- \(f\in \Theta(t(n)) \iff\) the cost (in terms of time or memory usage) of executing \(f\) increase as fast as \(t(n)\)
- \(f\in O(t(n)) \iff\) the cost (in terms of time or memory usage) of executing \(f\) increase slower than \(t(n)\)

If we have that \(f'\in O(f)\forall f\) solving \(X\) and \(f'\in \Theta(t'(n)))\), then we say \(X\) belongs to the complexity class of problems solvable with an algorithm with complexity \(t'(n)\), \(X\in C_{t'(n)}\).

Popular classes of interest include polinomial \(P\) and exponential \(E\).

#### Non-deterministic polynomial time class (NP)

A problem \(X\) belongs to the class \(N\!P\) if:

- the faster known solving algorithm for X is not polynomial, \(X\notin P\)
- given a solution \(s\) for \(X\), its correctness can be checked with a polynomial time algorithm.

### Reduction

Letâ€™s consider two problems \(X\) and \(Y\) with solving algorithms \(f_X,f_Y\) respectively.
If we can use \(f_X\) to solve \(Y\), it means problem \(Y\) **reduces** to \(X\) and, straightforwardly \(Y\) is at most difficult as \(X\).
One example to this scenario is given with \(X\) the problem of multiplying two numbers and \(Y\) the problem of computing the value of a squared number; the latter can always be solved using a solving algorithm for the former.

If we can do the same the other way round, using \(f_Y\) to solve \(X\), it means \(X\) is at most difficult as \(Y\) as well and \(X,Y\) are said to be **equivalent**.

### Hardness and completeness

Given a problem \(X\) and a complexity class \(C\), if every problem \(Y\in C\) can be reduced to \(X\) it means that \(X\) is hard at least as the hardest problem in \(C\) and so we say \(X\) is *\(C\)-hard*.

If \(X\) is \(C\)-hard and \(X\in C\) then \(X\) is said to be *\(C\)-complete*.

where last equality holds only if all problems \(Y\in C\) are equally difficult.

### \(P\) versus \(N\!P\)

While it is pretty straightforward that \(N\!P \supseteq P\), nor the equality or the proper (strict) inclusion have never been proved. Thus, we only have:

\[N\!P\text{-hard} \supset N\!P\text{-complete} \supseteq N\!P \supseteq P\]Leaving the hypothesis \(N\!P = P\) still not contradicted.