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(Requires calculus) Show that if \(b>1\) and \(c\) and \(d\) are positive, then \(\left(\log _{b} n\right)^{c}\) is \(O\left(n^{d}\right),\) but \(n^{d}\) is not \(O\left(\left(\log _{b} n\right)^{c}\right)\)

Short Answer

Expert verified
\( \left( \log_b n \right)^c \) is \( O(n^d) \), but \(n^d \) is not \( O(\left( \log_b n \right)^c) \).

Step by step solution

01

Define Big-O Notation

Recall that for a function \(f(n)\) to be \(O(g(n))\), there exist constants \(C > 0\) and \(n_0\) such that for all \(n \ge n_0\), \(f(n) \le Cg(n)\).
02

Evaluate \(\left(\log _{b} n\right)^{c}\)

Given \(b > 1\), the function \(\log_b n\) is increasing. Therefore, for large enough \(n\), \(\left(\log _{b} n\right)^{c}\) also increases.
03

Compare Growth Rates

Consider the growth rates of \(\left(\log_b n\right)^c\) and \(n^d\). As \(n\) becomes very large, \(n^d\) grows much faster than \(\left(\log_b n\right)^c\).
04

Show \( \left( \log_b n \right)^c \) is \(O(n^d)\)

Since \(n^d\) grows faster than \(\left( \log_b n \right)^c\), there exist constants \(C > 0\) and \(n_0\) such that \( \left( \log_b n \right)^c \le C n^d \) for all \( n \ge n_0 \). Hence, \( \left( \log_b n \right)^c \) is \(O(n^d)\).
05

Show \( n^d \) is not \(O(\left( \log_b n \right)^c)\)

Assume \( n^d \) is \(O(\left( \log_b n \right)^c)\). Then, there should exist constants \(C' > 0\) and \(n_0'\) such that \(n^d \le C' \left( \log_b n \right)^c\) for all \(n \ge n_0'\). However, as \(n \to \infty\), \( n^d \) grows faster than \( \left( \log_b n \right)^c\), violating the definition of Big-O notation. Thus, \( n^d \) is not \( O(\left( \log_b n \right)^c)\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

asymptotic analysis
Asymptotic analysis is a method used to describe the behavior of functions as inputs become large. It's a critical part of understanding algorithm efficiency, particularly in computer science.

The core idea is to focus on the dominant terms in a function to predict its growth rate and ignore lower-order terms. This helps in comparing different algorithms and understanding their performance.

For instance, if we want to compare \( f(n) = n^2 + 2n \) and \( g(n) = n^2 + 3 \), asymptotic analysis tells us that both grow similarly because the \( n^2 \) term dominates for large \( n \). Thus, they both belong to the Big-O class \( O(n^2) \).

  • Allows comparison of algorithm efficiencies
  • Focuses on dominant terms
  • Useful for predicting performance for large inputs
growth rates
Growth rates help us understand how functions behave as their inputs grow. In computer science, we particularly focus on how fast the runtime or space requirements of an algorithm increase relative to the input size.

There are different types of growth rates:
  • Constant (\( O(1) \)): The algorithm's performance does not change with input size.
  • Linear (\( O(n) \)): Performance increases linearly with the size of the input.
  • Quadratic (\( O(n^2) \)): Performance increases quadratically, making it slower for larger inputs.
  • Logarithmic (\( O(\log n) \)): Performance increases logarithmically, making it very efficient for large inputs.
  • Exponential (\( O(2^n) \)): Performance doubles with each additional element, making it inefficient for large inputs.
Understanding these growth rates helps in choosing or designing efficient algorithms.
logarithmic functions
Logarithmic functions are the inverse of exponential functions and are crucial in various fields of science and engineering.

A logarithmic function can be written as \( \log_b n \), where \( b \) is the base and \( n \) is the input. Logarithms are useful for describing phenomena that grow or shrink exponentially.

Key properties of logarithmic functions:
  • Logarithms convert multiplication into addition: \( \log_b(xy) = \log_b x + \log_b y \).
  • They convert division into subtraction: \( \log_b(x/y) = \log_b x - \log_b y \).
  • The base change formula: \( \log_b n = \log_k n / \log_k b \), for any base \( k \).
In computer science, dealing with large data sets often requires logarithmic functions for efficient searching and sorting algorithms like binary search.
exponential functions
Exponential functions grow rapidly and are expressed as \( f(n) = b^n \), where \( b \) is the base and \( n \) is the exponent. Because of their rapid growth, algorithms with exponential time complexity, like those involving \( 2^n \), are generally inefficient for large inputs.

Exponential growth can be seen in:
  • Population growth
  • Compound interest calculations
  • Algorithm complexities such as the Travelling Salesman Problem
In mathematics and science, understanding exponential functions helps in modeling and solving problems involving rapid changes such as viral spread or radioactive decay. Exponential functions are the antithesis of logarithmic functions, which grow very slowly.Recognizing when a problem has exponential complexity helps in making better decisions about algorithm or problem-solving strategies.

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Most popular questions from this chapter

There is a more efficient algorithm (in terms of the number of multiplications and additions used) for evaluating polynomials than the conventional algorithm described in the previous exercise. It is called Horner's method. This pseudocode shows how to use this method to find the value of \(a_{n} x^{n}+a_{n-1} x^{n-1}+\cdots+a_{1} x+a_{0}\) at \(x=c .\) $$ \begin{array}{c}{\text { procedure Horner }\left(c, a_{0}, a_{1}, a_{2}, \ldots, a_{n} : \text { real numbers) }\right.} \\ {y :=a_{n}} \\ {\text { for } i :=1 \text { to } n} \\ {y :=y * c+a_{n-i}} \\ {\text { return } y\left\\{y=a_{n} c^{n}+a_{n-1} c^{n-1}+\cdots+a_{1} c+a_{0}\right\\}}\end{array} $$ a) Evaluate \(3 x^{2}+x+1\) at \(x=2\) by working through each step of the algorithm showing the values assigned at each assignment step. b) Exactly how many multiplications and additions are used by this algorithm to evaluate a polynomial of degree \(n\) at \(x=c ?\) (Do not count additions used to increment the loop variable.)

Show that the problem of determining whether a program with a given input ever prints the digit 1 is unsolvable.

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In Exercises \(48-49\) assume that the number of multiplications of entries used to multiply a \(p \times q\) matrix and a \(q \times r\) matrix is \(p q r .\) What is the best order to form the product \(\mathbf{A B C}\) if \(\mathbf{A}\) , \(\mathbf{B},\) and \(\mathbf{C}\) are matrices with dimensions \(3 \times 9,9 \times 4,\) and \(4 \times 2,\) respectively?

Suppose that \(f(x), g(x),\) and \(h(x)\) are functions such that \(f(x)\) is \(\Theta(g(x))\) and \(g(x)\) is \(\Theta(h(x)) .\) Show that \(f(x)\) is \(\Theta(h(x)) .\)

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