Questions tagged [lower-bounds]

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Why decision tree method for lower bound on finding a minimum doesn't work

(Motivated by this question. Also I suspect that my question is a bit too broad) We know $\Omega(n \log n)$ lower bound for sorting: we can build a decision tree where each inner node is a comparison ...
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59 views

Counting circuits with constraints

Please forgive me if this question is trivial, I couldn’t come up with an answer (nor finding one). In order to show that there are boolean functions $f : \{0,1\}^n \rightarrow \{0,1\}$ which can be ...
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1answer
24 views

Lower bound on comparison-based sorting

I have a question from one of the exercises in CLRS. Show that there is no comparison sort whose running time is linear for at least half of the $n!$ inputs of length $n$. What about a fraction of $1/...
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Lower bounds for orthogonal matrix multiplication

Is it possible, according to the current state of knowledge, that orthogonal matrices can be multiplied faster than arbitrary matrices? More precisely, let $T(N)$ denote the worst-case time of the ...
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1answer
20 views

Communication Complexity for Product Distributions

In general for the (two-party) set disjointness problem for inputs of length n, we know that the parties need to communicate $\Omega(n)$. Surprisingly, today I discovered (if I understood correctly) ...
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2answers
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Algebra for min/max bounds

I am trying to model some set operations which are only well-defined if one is a subset of the other. The way the sets are constructed, I'll have a series of constraints of the form $x \subseteq y$, ...
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1answer
25 views

Lower bound time complexity for obtaining an arbitrary entry in a hashtable

I just answered this question on StackOverflow, which asks for an efficient algorithm such that given a nonempty hashtable, the algorithm should return a pointer to an arbitrary nonempty entry in the ...
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1answer
36 views

Decision tree lower bound for finding two array elements summing to zero

I have to solve this exercise: Given an unordered array $A[1], \ldots, A[n]$ of positive and negative integers, determine if there are two indices $i \neq j$ such that $A[i] + A[j] = 0$. ...
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1answer
12 views

Resolution exponential lower bound… alternative proofs?

I am reading the Resolution proof system exponential lower bound via Haken's bottleneck method for the Pigeonhole Principle as presented in Arora and Barak's Computational Complexity: A Modern ...
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30 views

Decision tree and information-theoretic lower bound

Consider the following problem : ...
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17 views

Finding the lower bound through decision trees

One way to find the lower bound of a comparison based algorithm is to use the decision tree. U have two questions regarding this method : 1) We know that the height of the tree is path that connects ...
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1answer
25 views

A Question related to the method of find lower bound : Trivial lower bounds

In Trivial lower bounds we just need to count the number of items in the input that needs to be processed and the number of items that need to be generated and the trivial lower bound time is then the ...
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50 views

Lower bound and worst case scenario

We know that the lower bound is the minimum amount of work needed to solve a problem. So for a given problem say x it has the best algorithm ( the most efficient algorithm to solve this problem ) say ...
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1answer
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Why there is no polynomially large sequence of polynomial large weights that derandomize the isolation lemma?

I was studying the paper Derandomizing the Isolation Lemma and Lower Bounds for Circuit Size by Arvind and Mukhopadhyay and came across the following claim (Observation 1.2 on page 3): "More ...
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What is an $O(n \log(n))$ binary sorting algorithm with a guaranteed low scaling constant on the run-time?

Let $O_c(f(n))$ denote that $c$ is the scaling constant for the run-time (e.g. $\text{run time} \leq c\cdot f(n) + B$ if $n$ is large enough) The absolute lower limit on the run-time for a binary ...
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61 views

Efficiently computing minimal elements over partially ordered sets

I have a list of sets that I would like to sort into a partial order based on the subset relation. In fact, I do not require the complete ordering, only the minimal elements. If I am not mistaken, ...
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1answer
51 views

Standard information-theoretic lower bound?

There should be a simple argument, but I'm struggling to see it. Suppose Alice has a string $x \in \{0, 1\}^n$ and sends a message $s = s(x)$ to Bob. And suppose that given $s$, Bob can reconstruct ...
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29 views

Is one probe to find a element is sufficient?

Let $M = \{1,2,3,\ldots,m\}$ be a a universe and $S \subseteq M$ denotes the number of elements comes from $M$ which we want to store such that query of the type "Is $x \in S$"?Let $n$ denotes the ...
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1answer
95 views

An example where the algorithm of Hopcroft and Karp performs poorly?

I have been trying to construct an example, where Hopcroft and Karp's algorithm for the maximum matching problem performs poorly (say at least $\Omega(\log n)$ rounds). However, all the examples I ...
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Why is the lower bound for sorting strings Ω(d + nlogn)?

I'm taking an Advanced Algorithms course. I'm currently studying efficient algorithms for sorting strings. In this chapter, it is provided a lower bound for the time complexity of $\Omega(d + n\log{n})...
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Conditional lower bounds on the running time of the single source shortest path problem

Just out of curiosity, I was wondering whether there is a conditional lower-bound on the running time of an algorithm for the Single Source Shortest Path Problem (on directed or undirected graphs). I ...
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2answers
283 views

Find both lower and upper asymptotic bounds for $T(n) = 2T(\frac{n}{2})+n^4$

So far we have learned Recursion Tree, Substitution Method, and Master's Theorem. I'm not sure how we can find lower AND upper bounds. I know that using Master's Theorem, we get $T(n) = \Theta(n^4)$, ...
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86 views

Decision Tree for searching an element in an n*n matrix

I just learnt decision tree concept in class. I have a question for homework. It says to prove that for searching an element in n*n matrix the lower bound is logn and prove it using decision tree. My ...
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Minimum amount of rectangles to create a 2-dimensional matrix

From this codegolf question. Consider an $r$ by $c$ matrix of nonnegative integers, called $M$. You also have a zero matrix of the same dimensions, called $N$. A "move" consists of replacing a ...
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1answer
32 views

Lower bound for merging $m$ sorted arrays (decision tree leaves count - permutations)

I need some help understanding how to calculate the lower bound on the time complexity of merging $m$ sorted arrays of length $n$. The bound should be $nm \lg(m)$. I need to prove this using a ...
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Useful conditions for proving super polynomial lower bound for some kind of recurrences

Given a recurrence of the form $\forall n,m.\ \ T(n,m)=\begin{cases}1,&,m=1\\\sum_i{T(n_i,m_i)}&,\text{else}\end{cases}$ Note: both $n_i$ and $m_i$ are dependent on $n,m$ so they should have ...
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Number of comparisons in array where each element appears n/k times [duplicate]

Given an array of $n$ elements with $k$ distinct elements, each appearing $n/k$ times, how can I show that the number of comparisons to the sort the array in the worst case is in $\Omega(n \log k)$?
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1answer
83 views

Lower Bound for Time Complexity of Pairing Problem

Given an array X and array Y both of length n, the pairing algorithm will return the elements of the arrays matched so that the smallest element in X will be matched with the smallest element of Y, ...
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1answer
53 views

Pebble game lower bound?

This paper says pebble games have super linear lower bound for every fixed $k$ https://dl.acm.org/citation.cfm?doid=62.322433. Why is it not considered proof of constructive example for a function in ...
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155 views

Dealing with test condition '=' for a while loop when determining a bound function/loop variant

The following is the definition of what a bound function for a while loop must satisfy: The bound function is an integer-valued, total function of some of the inputs, variables and global data that ...
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1answer
38 views

Asymptotics of a sinusoid

Consider the function $$ f(n) = 2n^2 |\sin(\pi \cdot n/2)|. $$ Which of the following classes does $f(n)$ belong to? $$ O(n^2), \Omega(n^2), \Theta(n^2), \omega(n^2), o(n^2). $$ I'm working in this ...
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1answer
182 views

Comparison-based lower-bound for finding duplicates in an array of $n$ numbers

Decision Problem: Given $n$ real numbers, give an algorithm that outputs "1" iff there are at least two numbers that are identical and outputs "0" otherwise. (Assume that comparison between any two ...
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96 views

Lower bound of disjointness by discrepancy?

I need to show that $Disc_\mu(Disj) \geq \frac{1}{2n+1}$ for any distribution $\mu: \{0,1\}^n \times \{0,1\}^n \to [0,1]$. Disjointness is defined as $Disj(X,Y)=\left\{ \begin{array}[ll]+1 & \...
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1answer
90 views

Why is finding minimum number of comparisons to sort $n$ elements so difficult?

In The Art of Computer Programming 2nd Ed, Vol 3, Section 5.3.1 then discuss a function $S(n)$ which is define as: $S(n)$ : The minimum number of comparisons that suffice to sort $n$ elements. ...
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1answer
34 views

Finding maximum takes at least $\lceil n/2 \rceil$ comparisons

We are given an array $A$ with $n$ elements, $n \in \mathbb{N}$ and all elements are in the set $\{1,2,3, \cdots, n \}$. I want to prove that finding the maximum in $A$ (that is, outputting the index ...
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1answer
3k views

What is an optimal algorithm?

I'm a computer science newbie and I thought I understood cases and bounds when I first studied them. I would take worst case as upper bound and best case as lower bound, but now I know that they are ...
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1answer
120 views

What is Ironic complexity? What are some good resources to learn about it?

The term "Ironic complexity" was coined by Scott Aaronson for the stuff Ryan Williams does in the area of complexity theory. Could anyone tell me what kind of problems and approaches does Ryan ...
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Postive interval problem lower bound

I was trying to solve the question given below. Algorithm : Using divide and conquer technique, divide the input till we get a very small size array's ( let us say of size 2 ). Solve these small ...
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Why do you need at least ln(n!) many comparison to sort a list?

"If every element comparison (testing whether $a_i \le a_j$ ) provides at most one bit of information, argue that you need at least on the order of $\ln(n!)$ many tests/comparisons to sort the list." ...
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1answer
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Do you >have< to define the upper and lower bound? (context: traveling salesman)

Do one have to define the upper and lower bound to be able to solve the tsp, or is that just an unnecessary intermediate step? And if so, why would one define those bounds? (context: the traveling ...
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2answers
190 views

Why are greedy algorithms used to find upper/lower bounds? (when they doesn't guarantee an optimal solution)

Take the nearest neighbor algorithm for the traveling salesman problem as an example. Why is it used to find the upper bound? When can't it guarantee an optimal solution? (Thanks to many comments ...
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254 views

Does finding a cycle with $\log n$ length in $\text{P}$?

Let $G$ be an arbitrary graph with $n$ vertices and we want to find a simple cycle with $\log n$ length. Is there exists a known polynomial algorithm for this problem?
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1answer
145 views

Using Yao's principle to find a lower bound

This is a HW question, so I'm not expecting any answers, just a general guidance/help. Definition. Given $\underset{\neq0}{\underbrace{s}}\in\left\{ 0,1\right\} ^{n}$, a function $f:\left\{ 0,1\right\...
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1answer
42 views

Showing $2^x$ is a lower bound

How do I show that $2^x - x^2 \in \Omega(2^x)$? Basically, I know that this means that $\exists a, x_0 \in \mathbb{R^+}, \forall x \in \mathbb{N}, a.2^x \leq 2^x - x^2$. I worked around a bit with ...
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findMax reduce to sort

Can I reduce the find max (or find min) problem to the sort problem? Because if so, knowing the lower bound for find max is Ω(n) I can also infer that the lower bound for sorting is Ω(n) too? I'm ...
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1answer
241 views

lower bound proof with adversary argument

We have to run a song on a Walkman, for that we need 2 full batteries. Let's say we have a mixed set of 30 batteries (15 are empty and and 15 are full) and then only way to test if the battery is full ...
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Query complexity of exact learning and combinatorial parameter

When defining the query complexity of exact learning for a concept $c$ (considered as a function from $\{0,1\}^n \mapsto \{0,1\}$) in a concept class $\mathcal{C}$, we often come across the following ...
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1answer
93 views

Minimum and maximum of sum of inverse degree of a graph

Suppose we have a simple undirected graph $G(V,E)$, where $V$ and $E$ are the set of vertices and edges respectively. we denote $d(v)$ as the degree of a vertex $v \in V$. I am interested to find ...
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Highest lower bound on an NP complete problem

What is the highest time complexity lower bound that has been proven on any (non-contrived) NP complete problem?
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1answer
333 views

Correctness of lower bound proof

I am working on this exercise with the purpose of learning how to provide proper proofs and I would like to know if my proof for the following problem is correct. Given a sorted array $A$ (of $n$ ...