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# Strassen’s Algorithm MCQ’s

Strassen’s algorithm is a/an_____________ algorithm.

a) Non- recursive

b) Recursive

c) Approximation

d) Accurate

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Answer: b

Explanation: Strassen’s Algorithm for matrix multiplication is a recursive algorithm since the present output depends on previous outputs and inputs.

What is the running time of Strassen’s algorithm for matrix multiplication?

a) O(n^{2.81})

b) O(n^{3})

c) O(n^{1.8})

d) O(n^{2})

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Answer: a

Explanation: Strassen’s matrix algorithm requires only 7 recursive multiplications of n/2 x n/2 matrix and Theta(n^{2}) scalar additions and subtractions yielding the running time as O(n^{2.81})

What is the running time of naïve matrix multiplication algorithm?

a) O(n^{2.81})

b) O(n^{4})

c) O(n)

d) O(n^{3})

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Answer: d

Explanation: The traditional matrix multiplication algorithm takes O(n^{3}) time. The number of recursive multiplications involved in this algorithm is 8

Strassen’s matrix multiplication algorithm follows ___________ technique.

a) Greedy technique

b) Dynamic Programming

c) Divide and Conquer

d) Backtracking

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Answer: c

Explanation: Strassen’s matrix multiplication algorithm follows divide and conquer technique. In this algorithm the input matrices are divided into n/2 x n/2 sub matrices and then the recurrence relation is applied

The number of scalar additions and subtractions used in Strassen’s matrix multiplication algorithm is ________

a) O(n^{2.81})

b) Theta(n^{2})

c) Theta(n)

d) O(n^{3})

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Answer: b

Explanation: Using Theta(n^{2}) scalar additions and subtractions, 14 matrices are computed each of which is n/2 x n/2. Then seven matrix products are computed recursively.

Running time of Strassen’s algorithm is better than the naïve Theta(n^{3}) method.

a) True

b) False

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Answer: a

Explanation: Strassen’s Algorithm requires only 7 recursive multiplications when compared with the naïve Theta(n^{3}) method which reuires 9 recursive multiplications to compute the product.

Given the program of naïve method.

for i=1 to n do for j=1 to n do Z[i][j]=0; for k=1 to n do ___________________________

Fill in the blanks with appropriate formula

a) Z[i][j] = Z[i][j] + X[i][k]*Y[k][j]

b) Z[i][j] = Z[i][j] + X[i][k] + Y[k][j]

c) Z[i][j] = Z[i][j] * X[i][k]*Y[k][j]

d) Z[i][j] = Z[i][j] * X[i][k] + Y[k][j]

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Answer: a

Explanation: In the naïve method of matrix multiplication the number of iterating statements involved are 3, because of the presence of rows and columns. The element in each row of one matrix is multiplied with each element in the column of the second matrix. The computed value is placed in the new matrix Z[i][j].

Who demonstrated the difference in numerical stability?

a) Strassen

b) Bailey

c) Lederman

d) Higham

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Answer: d

Explanation: The difference in the numerical stability was demonstrated by Higham. He overemphasized that Strassen’s algorithm is numericaly unstable for some applications.

What is the recurrence relation used in Strassen’s algorithm?

a) 7T(n/2) + Theta(n^{2})

b) 8T(n/2) + Theta(n^{2})

c) 7T(n/2) + O(n^{2})

d) 8T(n/2) + O(n^{2})

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Answer: a

Explanation: The recurrence relation used in Strassen’s algorithm is 7T(n/2) + Theta(n^{2}) since there are only 7 recursive multiplications and Theta(n^{2}) scalar additions and subtractions involved for computing the product.

Who discussed techniques for reducing the memory requirements for Strassen’s algorithm?

a) Strassen

b) Lederman

c) Bailey

d) Higham

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Answer: c

Explanation: The submatrices formed at the levels of recursion consume space. Hence in order to overcome that Bailey discussed techniques for reducing the memory required.

What is the formula to calculate the element present in second row, first column of the product matrix?

a) M1+M7

b) M1+M3

c) M2+M4 – M5 + M7

d) M2+M4

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Answer: d

Explanation: The element at second row, first column can be found by the formula M2 + M4, where M2 and M4 can be calculated by

M2= (A(2,1) + A(2,2)) B(1,1)

M4=A(2,2)(B(1,2) – B(1,1)).

Strassen’s Matrix Algorithm was proposed by _____________

a) Volker Strassen

b) Andrew Strassen

c) Victor Jan

d) Virginia Williams

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Answer: a

Explanation: Strassen’s matrix multiplication algorithm was first published by Volker Strassen in the year 1969 and proved that the n^{3} general matrix multiplication algorithm wasn’t optimal

How many iterating statements are involved in the naïve method of matrix multiplication?

a) 1

b) 2

c) 3

d) 4

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Answer: c

Explanation: In the naïve method of matrix multiplication the number of iterating statements involved are 3, because of the presence of rows and columns in a matrix. The element in each row of the first matrix is multiplied with each element in the column of the second matrix.

Strassen’s algorithm is quite numerically stable as the naïve method.

a) True

b) False

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Answer: b

Explanation: Strassen’s algorithm is too numerically unstable for some applications. The computed result C=AB satisfies the inequality with a unit roundoff error which corresponds to strong stability inequality(obtained by replacing matrix norms with absolute values of the matrix elements).

Compute the product matrix using Strassen’s matrix multiplication algorithm.

Given a11=1; a12=3;a21=5;a22=7

b11=8;b12=4;b21=6;b22=2

a) c11=20;c12=12;c21=100;c22=15

b) c11=22;c12=8;c21=90;c22=32

c) c11=15;c12=7;c21=80;c22=34

d) c11=26;c12=10;c21=82;c22=34

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Answer: d

Explanation: The solution can be obtained by

C11=1*8 + 3*6 =8+18=26

C12=1*4 + 3*2 =4+6=10

C21=5*8 + 7*6 =40+42=82

C22= 5*4 + 7*2=20+14=34.