Linked Questions

2
votes
0answers
143 views

Why are optimization problems always NP-hard and not NP-complete and what does this mean for other levels of the polynomial time hierarchy? [duplicate]

I have read that optimization problems cannot be $\mathcal{NP}$-complete, but are always classified as $\mathcal{NP}$-hard. When a problem is NP-complete, I know it is contained in $\mathcal{NP}$P. ...
283
votes
7answers
131k views

What is the definition of P, NP, NP-complete and NP-hard?

I'm in a course about computing and complexity, and am unable to understand what these terms mean. All I know is that NP is a subset of NP-complete, which is a subset of NP-hard, but I have no idea ...
37
votes
3answers
4k views

Decision problems vs “real” problems that aren't yes-or-no

I read in many places that some problems are difficult to approximate (it is NP-hard to approximate them). But approximation is not a decision problem: the answer is a real number and not Yes or No. ...
28
votes
2answers
9k views

Optimization version of decision problems

It is known that each optimization/search problem has an equivalent decision problem. For example the shortest path problem optimization/search version: Given an undirected unweighted graph $G ...
11
votes
2answers
1k views

Why are decision problems commonly used in complexity theory?

From Wikipedia: The type of computational problem: The most commonly used problems are decision problems. However, complexity classes can be defined based on function problems, counting ...
-2
votes
2answers
2k views

complexity of decision problems vs computing functions [closed]

This is an area that admittedly I've always found subtle about CS and occasionally trips me up, and clearly others. recently on tcs.se a user asked an apparently innocuous question about N-Queens ...
5
votes
0answers
663 views

Time complexity of finding the largest factor of a number (using a specific oracle)

My question is related to this question posted on math.SE: Given an odd number, what is the quickest (constant-time) algorithm for finding its largest factor and suppose you can call a helper ...
1
vote
0answers
352 views

Does FNP-complete = NP-complete?

I can't seem to find this stated explicitly anywhere, which makes me wonder if I have it all wrong. So first, let's say we view problems in NP as degenerate problems in FNP, where the codomain of the ...
1
vote
1answer
176 views

What is the usage of a decision problem for an optimization problem like the longest path problem?

I just read this definition for the longest path problem: LONGEST PATH Input: A graph $G=(V,E)$, an integer $k$. Question: Is there a path with at least $k$ vertices in $G$ This seems a ...
1
vote
1answer
45 views

Reframing decision, counting, enumeration, and search as optimization

The top accepted answers to the questions below allude to two complexity classes of optimization problems: NPO and PO (in relation to NP and P for decision problems): Decision problems vs "real" ...