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Professor F. Lake tells his class that it is asymptotically faster to square an -bit integer than to multiply two n-bit integers. Should they believe him?

Short Answer

Expert verified

No, the statement of Professor F. Lake is wrong.

Step by step solution

01

Introduction

Evaluate the following details:

• The algorithm's execution time is determined using asymptotic notations.

• According to the Professor, the n-bit integer algorithm squaring is faster than multiplying two n-bit integers.

• The algorithm's asymptotic value is determined by the number of items and actions in the algorithm.

02

Correct Fact

“No”, students should not believe the professor.

Description:

• When an n-bit integer is squared, numerous cross-terms become equal.

• As a result, there's no need to do it again.

• Take the 3-bit integer "1012" as an example.

• This number is written as a2, a1 , and a0 from left to right.

• If you square this number, you'll obtain 9 terms.

• In the matrix below, the terms are listed.

a2a2a2a1a2a0a1a2a1a1a1a0a0a2a0a1a0a0

• This is a symmetric matrix. Because the primary diagonal's components are all the same.

• In other words, certain elements do not require calculation.

• As a result, this approach for squaring n-bit values is not asymptotically superior.

• This algorithm will benefit from a steady pace.,

As a result, Professor is incorrect in his assertion that the fast-squaring method may be improved at a constant pace.

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Most popular questions from this chapter

An array A[1...n] is said to have a majority element if more than half of its entries are the same. Given an array, the task is to design an efficient algorithm to tell whether the array has a majority element, and, if so, to find that element. The elements of the array are not necessarily from some ordered domain like the integers, and so there can be no comparisons of the form “ is A[i]>A[j] ?”. (Think of the array elements as GIF files, say.) However you can answer questions of the form: “is ..?” in constant time.

(a) Show how to solve this problem in O(nlogn) time. (Hint: Split the array A into two arrays A1and A2of half the size. Does knowing the majority elements of A1and A2help you figure out the majority element of A? If so, you can use a divide-and-conquer approach.)

(b) Can you give a linear-time algorithm? (Hint: Here’s another divide-and-conquer approach:• Pair up the elements of A arbitrarily, to get n/2 pairs• Look at each pair: if the two elements are different, discard both of them; if they are the same, keep just one of them . Show that after this procedure there are at most n/2 elements left, and that they have a majority element if A does.)

Practice with the fast Fourier transform.

(a) What is the FFT of (1,0,0,0)? What is the appropriate value of ωin this case? And of which sequence is (1,0,0,0)the FFT?

(b)Repeat for (1,0,1,-1).

Suppose we want to evaluate the polynomial P(x) = a0 + a1x + a2x2 + ... + anxn at point x.

  1. Show that the following simple routine, known as Horner’s rule, does the job and leaves the answer in z.
    z = an
    for I = n-1 down to 0 :
    z = zx + ai
  2. How many additions and multiplications does this routine use, as a function of n ? Can you find a polynomial for which an alternative method is substantially better?

Question: On page 66 there is a high-level description of the quicksort algorithm.

(a) Write down the pseudocode for quicksort.

(b) Show that its worst - case running time on an array of size n is Θ(n)2.

(c) Show that its expected running time satisfies the recurrence relation.

T(n)O(n)+1ni=1n-1(Ti+Tn-i)

Then, show that the solution to this recurrence is O(nlogn).

In this problem we will develop a divide-and-conquer algorithm for the following geometric task.

CLOSEST PAIRInput: A set of points in the plane, {p1=(x1;y1),p2=(x2,y2),...,pn=(xn,yn)}

Output: The closest pair of points: that is, the pair PiPjfor which the distance between piand pj, that is,

(xi-xi)2+z(yi-yi)2,

is minimized.

For simplicity, assume that n is a power of two, and that all the x-coordinates role="math" localid="1659237354869" xi are distinct, as are the y-coordinates.

Here’s a high-level overview of the algorithm:

.Find a value for which exactly half the points have xi<x, and half have xi>x. On this basis, split the points into two groups, L and R.

• Recursively find the closest pair in L and in R. Say these pairs are pL·qLLand pR·qRRwith distances dLand dR respectively. Let d be the smaller of these two distances.

• It remains to be seen whether there is a point in Land a point in R that are less than distance dapart from each other. To this end, discard all points with xi<x-dor xi>x+d and sort the remaining points by y-coordinate.

• Now, go through this sorted list, and for each point, compute its distance to the seven subsequent points in the list. Let pM·qMbe the closest pair found in this way.

• The answer is one of the three pairs role="math" localid="1659237951608" {pL,qL},{pR,qR}{pM,qM}, whichever is closest.

(a) In order to prove the correctness of this algorithm, start by showing the following property: any square of size d×d in the plane contains at most four points of L.

(b) Now show that the algorithm is correct. The only case which needs careful consideration is when the closest pair is split between L and R.

(c) Write down the pseudocode for the algorithm, and show that its running time is given by the recurrence:

T(n)=2T(nl2)+0(nlogn)

Show that the solution to this recurrence is o(nlogzn).

(d) Can you bring the running time down to O(nlogn)?

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