Math Problem Statement

Find a solution to the following divide-and-conquer recurrence relations. You may use any method.

For all problems, assume that T(1) = 1.

(1) (Home Work) T(n) = 3T(n/2) + n lg n.

T(1) = 1, T(n) = 3T(n/2) + n log n

let n = 2^k, k = 0,1,2,3,...,infinity

T(2^k) = 3T((2^K)/2) + 2^K (log{2^k})

= 3T(2^{k-1}) + 2k^2 (k) (log{2})

= 3T(2^{k-1}) + k2^k

3T(2^k) - T(2^{k-1}) = k2^k

(x-3)(x-2) = 0

r_1 = 3, r_2 =2

general solution: T(2^k) = c_1(3^k) + A(2^k)

finding constants from boundary conditions

k = 0 => T(2^0) = T(1) = 1

k = 1 => T(2^1) = 3T(1) + 2 (log(2)) = 5

k = 0 => c_1 (3^0) + A(2^0) = 1

k = 1 => c_1 (3^1) + A(2^1) = 5

solving for the constants c_1 = 3, A = -2

T(2^k) = 3(3^k) + -2(-2^k)

since n=2^k => k = log_2{n}

T(n) = 3(3^{log_2 n}) + -2(2^{log_2} n)

is it correct? if not then correct it

Solution

Let's solve the recurrence relation T(n)=3T(n/2)+nlognT(n) = 3T(n/2) + n \log n with the base case T(1)=1T(1) = 1 using the Master Theorem.

Step 1: Identify the Components

The recurrence has the form:

T(n)=aT(nb)+f(n)T(n) = aT\left(\frac{n}{b}\right) + f(n)

where:

  • a=3a = 3
  • b=2b = 2
  • f(n)=nlognf(n) = n \log n

Step 2: Calculate nlogban^{\log_b a}

We first compute logba\log_b a:

logba=log23\log_b a = \log_2 3

This means:

nlogba=nlog23n^{\log_b a} = n^{\log_2 3}

Step 3: Compare f(n)f(n) with nlogban^{\log_b a}

Next, we need to compare f(n)=nlognf(n) = n \log n with nlog23n^{\log_2 3}:

  1. Finding log23\log_2 3: Since log23\log_2 3 is approximately 1.5851.585, we have:

nlog23=n1.585n^{\log_2 3} = n^{1.585}

  1. Comparing nlognn \log n and n1.585n^{1.585}:

We see that:

f(n)=nlogn=o(nlog23)as nf(n) = n \log n = o(n^{\log_2 3}) \quad \text{as } n \to \infty

This means f(n)f(n) grows slower than nlogban^{\log_b a}.

Step 4: Apply the Master Theorem

Since f(n)f(n) is polynomially smaller than nlogban^{\log_b a}:

f(n)=o(nlogbaϵ)for some ϵ>0f(n) = o\left(n^{\log_b a - \epsilon}\right) \quad \text{for some } \epsilon > 0

We can apply Case 1 of the Master Theorem:

T(n)=Θ(nlog23)T(n) = \Theta(n^{\log_2 3})

Step 5: Final Result

Thus, we conclude:

T(n)=Θ(nlog23)=Θ(n1.585)T(n) = \Theta(n^{\log_2 3}) = \Theta(n^{1.585})

Summary

Your approach started correctly with the recurrence relation but had some errors in the derivation of constants and final expressions. The correct solution is:

T(n)=Θ(nlog23)Θ(n1.585)T(n) = \Theta(n^{\log_2 3}) \approx \Theta(n^{1.585})

Would you like more details on any specific part of the solution? Here are some related questions:

  1. What are the implications of the result for algorithms using this recurrence?
  2. How would you solve a similar recurrence with different coefficients?
  3. Can you explain the different cases of the Master Theorem?
  4. How does the f(n)f(n) term affect the overall complexity?
  5. What would happen if f(n)f(n) were instead n2n^2?

Ask a new question for Free

By Image

Drop file here or Click Here to upload

Math Problem Analysis

Mathematical Concepts

Recurrence Relations
Divide and Conquer
Asymptotic Analysis

Formulas

T(n) = aT(n/b) + f(n)
n^{log_b a}
Master Theorem

Theorems

Master Theorem

Suitable Grade Level

Grades 10-12