Questions tagged [approximation]

Questions about algorithms that solve problems up to some bounded error.

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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. ...
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31 votes
7 answers
39k views

Algorithm to distribute items "evenly"

I'm searching for an algorithm to distribute values from a list so that the resulting list is as "balanced" or "evenly distributed" as possible (in quotes because I'm not sure these are the best ways ...
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  • 433
23 votes
3 answers
2k views

Why are NP-complete problems so different in terms of their approximation?

I'd like to begin the question by saying I'm a programmer, and I don't have a lot of background in complexity theory. One thing that I've noticed is that while many problems are NP-complete, when ...
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  • 505
23 votes
3 answers
2k views

Approximating the Kolmogorov complexity

I've studied something about the Kolmogorov Complexity, read some articles and books from Vitanyi and Li and used the concept of Normalized Compression Distance to verify the stilometry of authors (...
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22 votes
0 answers
512 views

Approximate minimum-weighted tree decomposition on complete graphs

Say I have a weighted undirected complete graph $G = (V, E)$. Each edge $e = (u, v, w)$ is assigned with a positive weight $w$. I want to calculate the minimum-weighted $(d, h)$-tree-decomposition. By ...
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  • 321
19 votes
3 answers
3k views

Why there are no approximation algorithms for SAT and other decision problems?

I have an NP-complete decision problem. Given an instance of the problem, I would like to design an algorithm that outputs YES, if the problem is feasible, and, NO, otherwise. (Of course, if the ...
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  • 673
18 votes
2 answers
12k views

PTAS definition vs. FPTAS

From what I read in the ...
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14 votes
1 answer
306 views

How can you bound the error of an approximation without knowing the optimal solution?

I been looking at this site and it says that people found solutions for TSP tours that are just 0.031% higher than the optimal tour is. Without finding the optimal tour how does they know what length ...
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14 votes
1 answer
312 views

Approximation of minimum bandwidth on binary trees

Minimum bandwidth problem is to a find an ordering of graph nodes on integer line that minimizes the largest distance between any two adjacent nodes. The decision problem is NP-complete even for ...
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13 votes
1 answer
17k views

What does the 2 in a 2-approximation algorithm mean?

Does the 2 in a 2-approximation algorithm mean the solution is within 2*OPT or OPT/2?
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13 votes
1 answer
1k views

Why are all problems in FPTAS also in FPT?

According to the Wikipedia article on polynomial-time approximation schemes: All problems in FPTAS are fixed-parameter tractable. This result surprises me - these classes seem to be totally ...
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11 votes
2 answers
4k views

What is a bicriteria approximation algorithm?

What is a bicriteria approximation algorithm? This keeps coming up in the case of data stream clustering. Is this related to multi-objective optimization? This is where I came across it: cis.upenn....
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11 votes
2 answers
178 views

Does #$P$-Completeness imply approximation hardness?

Let $\Pi$ be some counting problem which is known to be #$P$-Complete. Does it imply that $\Pi$ is $APX$-hard (i.e. no PTAS for the problem exists unless $P=NP$)?
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11 votes
1 answer
1k views

Average length of s-t (simple) paths in a directed graph

Given the fact that $s$-$t$ path enumeration is a #P-complete problem, could there be efficient methods that compute (or at least approximate) the average length of $s$-$t$ path without enumerating ...
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10 votes
3 answers
10k views

Difference between heuristic and approximation algorithm?

i have a problem regarding the following situation. I have two arrays of numbers like this: ...
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10 votes
2 answers
4k views

Knapsack Greedy Approximation: Worst Case

I am currently studying approximation algorithms and I have run into an issue with a study problem. The approximation algorithm is for the general Knapsack problem, and it proposes a greedy approach, ...
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10 votes
1 answer
326 views

Hardness of approximating 0-1 integer programs

Given a $0,1$ (binary) integer program of the form: $$ \begin{array}{lll} \text{min} & f(x) & \\ \text{s.t.} & A x = b \\ & x_i \ge 0 & \quad \forall i\\ & x_i \in \{0,1\} &...
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10 votes
1 answer
178 views

Mathematical optimization on a noisy function

Let $f:\mathbb{R}^d \to \mathbb{R}$ be a function that is fairly nice (e.g., continuous, differentiable, not too many local maxima, maybe concave, etc.). I want to find a maxima of $f$: a value $x \...
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10 votes
2 answers
251 views

Is this combinatorial optimisation problem similar to any known problem?

The problem is as follows: We have a two dimensional array/grid of numbers, each representing some "benefit" or "profit." We also have two fixed integers $w$ and $h$ (for "width" and "height".) And a ...
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10 votes
0 answers
155 views

Complexity class for probabilistic approximation algorithms with bounded error

What's the name of a complexity class of optimization problems that have "bounded error probabilistic approximation algorithms"? Bounded error probabilistic version of APX (as BPP is bounded error ...
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  • 560
9 votes
2 answers
2k views

Looping and branching with Algorithmic Differentiation

Algorithmic (aka Automatic) Differentiation is a wonderful technique for numerical computation of derivatives. I understand how it relates to the fact that we know how to deal with every elementary ...
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  • 205
9 votes
2 answers
235 views

Find $\epsilon'$ s.t $L_\epsilon$ is $\mathsf{NP}$-hard for any $\epsilon<\epsilon'$

Let $L_\epsilon$ be the language of all $2$-CNF formulas $\varphi$, such that at least $(\frac{1}{2}+\epsilon)$ of $\varphi$'s clauses can be satisfied. I need to prove that there exists $\epsilon'$ ...
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  • 511
9 votes
1 answer
2k views

How do GPUs compute sines?

I've been wondering lately how GPUs compute sines and cosines, and Google hasn't helped me finding a precise answer. Initially, I was thinking that in order to make the computations as fast as ...
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  • 193
9 votes
2 answers
658 views

Prove that the 2-approximation of a modified local search algorithm for max-cut is tight

Consider the following local search approximation algorithm for the unweighted max cut problem: start with an arbitrary partition of the vertices of the given graph $G = (V,E) $, and as long as you ...
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  • 113
9 votes
1 answer
578 views

Hardness of approximating Minimum Cardinality Exact Cover

The Minimum Cardinality Exact Cover (MCEC) problem is just like set cover, but the output sets must be disjoint. Formally, given a collection of subsets $S$ of a finite set $U$, the problem asks for ...
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9 votes
1 answer
363 views

2D interval scheduling problem

Suppose I give you $n$ axis-aligned rectangles with a specified width, height, and x-position (of the left edge) $\{(w_i, h_i, x_i) \mid i \in \{0, \ldots, n - 1\}\}$, as well as a bound $(y_\mathrm{...
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  • 364
8 votes
3 answers
654 views

What is the name of this logistic variant of TSP?

I have a logistic problem that can be seen as a variant of $\text{TSP}$. It is so natural, I'm sure it has been studied in Operations research or something similar. Here's one way of looking at the ...
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8 votes
2 answers
3k views

Why is Savage's Vertex Cover algorithm a 2-approximation?

Carla. D. Savage formulated the following approximation algorithm for the vertex cover problem. Given graph $G$, start at arbitrary node and traverse $G$ depth-first Obtain DFS tree $T$ return $V_C =$...
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  • 343
8 votes
2 answers
3k views

Providing Tight Example in Approximation Algorithm Analysis

Let's say I found a 2-approximation algorithm for a certain problem and I want to show that the analysis is tight. Do I now need to come up with an example of generic size $n$ or does it suffice to ...
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  • 81
8 votes
1 answer
114 views

What is the approximation ratio of this bin-backing algorithm?

Consider the following algorithm for bin packing: Initially, sort the items by their size. Put the largest item in a new bin. Fill the bin with small items in ascending order of size, up to the ...
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8 votes
1 answer
6k views

Balanced Weight Distribution in Bins/Buckets

Let $W = \{w_1,w_2,...w_n\}$ be a set of integer weights. Let $B = \{b_1,b_2,...b_m\}$ be a set of buckets, with $m \leq n$. Let $T(b_j)$ represent the total weight present in bucket $b_j$, which is ...
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8 votes
1 answer
510 views

The heaviest induced subgraph problem

I am interested in such a combinatorial problem: given a graph $G=(V, E)$ and a weight functions $w_v: V \mapsto R$, and $w_e: E \mapsto R$ we are asking about such a induced subgraph $G' = (V', E')$ ...
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8 votes
0 answers
156 views

Compute the expected size of an approximation of vertex cover

Consider the following randomized approximation algorithm of vertex cover: Input: A graph G = (V, E). Output: A set $C_G \subseteq V$ a vertex cover of $G$. The algorithm: Set $C_G := \emptyset$. ...
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8 votes
0 answers
86 views

Connections between circuit complexity and Unique Games Conjecture?

Circuit complexity has connections to many questions in complexity theory. For a couple examples, Ryan Williams shared some in a recent talk and Section 3 of these notes gives simple relations to $\...
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8 votes
0 answers
1k views

Weighted Maximum 3-DIMENSIONAL-MATCHING with restricted weights (Approx Algo)

If the weights of the weighted 3-DIMENSIONAL-MATCHING problem are restricted to let's say, 1 and 2, is there a possibility to reduce this case to the unweighted 3-DIMENSIONAL-MATCHING problem? (...
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7 votes
3 answers
2k views

What is the fastest algorithm to approximate an irrational number with specified precision?

Problem Background: Let $a\in(0,1)$ to be an irrational number. Suppose there is a black box, the input is a real number in $[0,1]\backslash \{a\}$, denoted as $x$, the black box outputs boolean ...
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7 votes
2 answers
1k views

Why does this graph show the tightness of MST heuristic's 2-approximation bound?

This is a homework problem I've been given and I've been raking my brain for hours (so I'm satisfied with some pointers). I know already that the approximation ratio cannot be worse than $2$. I have a ...
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7 votes
1 answer
1k views

Approximation algorithm for Feedback Arc Set

Given a directed graph $G = (V,A)$, a feedback arc set is a set of arcs whose removal leaves an acyclic graph. The problem is to find the minimum cardinality such set. I want to find out about is ...
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7 votes
1 answer
447 views

Are there any problems in $APX - PTAS$ that are not $APX$-complete?

I have a question about the structure of the complexity class $APX$. Obviously, unless $P=NP$, no problem in the class $PTAS$ can be $APX$-complete (under the AP-reduction). However, what about the ...
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  • 686
7 votes
1 answer
416 views

Approximation algorithms for NP-complete problems

Given two NP NP-hard functional problems, A and B, one can find a reduction of A to B. Is it possible to find a reduction that would honour approximations? That is, if you have an approximation ...
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  • 560
7 votes
1 answer
311 views

Does $\#W$[1]-hardness imply approximation hardness?

Let $\Pi$ be a parametrized counting problem, where the parameter is the solution cost, e.g. counting the number of $k$-sized vertex cover in a graph, parametrized by $k$. Assume that $\Pi$ is $\#W$[...
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  • 2,594
7 votes
1 answer
483 views

Inapproximability result implies apx-hardness?

If an optimization problem is known to be inapproximable up to some precision, does this automatically imply that the problem is apx-hard?
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  • 145
7 votes
1 answer
418 views

In s-t directed graph, how to find many small cuts?

Solving the maximum flow problem yields one qualified minimal cut. But I want several (maybe hundreds) small cuts as candidates. The cuts don't have to be minimum cuts, as long as they are small (in ...
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  • 123
7 votes
1 answer
2k views

Line smoothing algorithm that perserve data uniformity

Intro: I'm working with huge data set that i need to plot in browser, and since there may be up to 1M points my idea was to create different representations for different zoom levels lets say i have ...
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  • 171
6 votes
2 answers
1k views

Difference between approximation scheme and approximation algorithm?

What is the difference between approximation schemes and approximation algorithms? Why do we study approximation schemes?
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  • 83
6 votes
1 answer
866 views

$1+\epsilon$ approximation for inapproximable problems

I am currently confused by the following situation: 1) The metric $k$-center problem is inapproximable in polynomial time within $2-\epsilon$ unless $P=NP$. 2) The metric $k$-center problem can ...
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  • 63
6 votes
2 answers
970 views

Example for a non-trivial PCP verifier for an NP-complete problem

During my involvement in a course on dealing with NP-hard problems I have encountered the PCP theorem, stating $\qquad\displaystyle \mathsf{NP} = \mathsf{PCP}(\log n, 1)$. I understand the ...
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  • 71k
6 votes
1 answer
328 views

MAXSAT approximation

We have been studying a 1/2-approximation for MAXSAT which runs in expected polynomial time, by randomly assigning True/False to each variable and repeating until we reach an assignment with at least ...
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  • 344
6 votes
2 answers
3k views

What is inapproximability of NP-hard problems?

Recently I have come across a paper which talks of "(1+ε)-inapproximability" and of "logarithmic approximation". While I have a basic knowledge of computational complexity (I more or less know what ...
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6 votes
1 answer
2k views

Why it is nearly impossible to have an approximation algorithm for Maximum Clique problem?

I read a theorem which states that: If there exists a polynomial time approximation algorithm for solving the Maximum Clique problem (or the Maximum Independent Set problem) for any constant ...
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