A Matrix Problem

Algebra Level 3

Read the following statements carefully.

( 1 (1 ) The elements on the main diagonal of a symmetric matrix are all zero.

( 2 (2 ) The elements on the main diagonal of a skew-symmetric matrix are all zero.

( 3 (3 ) For any square matrix A , 1 2 ( A + A ) A, \dfrac{1}{2}(A + A') is skew-symmetric.

( 4 (4 ) For any square matrix A , 1 2 ( A A ) A, \dfrac{1}{2}(A - A') is skew-symmetric.

Which of the following statements are correct?

Only 3 , 4 3, 4 Only 2 , 4 2, 4 Only 2 , 3 , 4 2, 3, 4 Only 1 , 2 1, 2

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4 solutions

Aman Sharma
Nov 26, 2014

I think statement 3 is incorrect:- For a square matrix A A , 1 2 ( A + A ) \frac{1}{2}(A+A') is a symetric matrix not skew symetric matrix:- Consider a square matrix A A :- Also let:- 1 2 ( A + A ) = K . . . . . . . . . ( 1 ) \frac{1}{2}(A+A')=K.........(1) Taking transpose on both sides of equation (1):- 1 2 ( A + A ) = K . . . . . . ( 2 ) \frac{1}{2}(A+A')'=K'......(2) Now using the properties well known properties of transpose operator:- 1 2 ( A + A ) = 1 2 ( A + ( A ) ) = 1 2 ( A + A ) . . . . . ( 3 ) \frac{1}{2}(A+A)'=\frac{1}{2}(A'+(A')')=\frac{1}{2}(A+A').....(3) Now putting the value of 1 2 ( A + A ) \frac{1}{2}(A+A')' from equation (3) in (2):- 1 2 ( A + A ) = K . . . . . ( 4 ) \frac{1}{2}(A+A')=K'.....(4) Clearly LHS of equations (1) and (A) are same hence there RHS must be same so:- K = K K=K' Putting the value of K in above equation:- 1 2 ( A + A ) = 1 2 ( A + A ) \frac{1}{2}(A+A')=\frac{1}{2}(A+A')' which is defination of symetric matrix not of skew symetric matrix So i think question may be wrong........it is also possible that i am doing something wrong ,can someone please explain why is statement 3 correct

Very easy Question !!( at most level-2 problem )

Karan Shekhawat - 6 years, 6 months ago

I agree. Statement 3 is not correct

Parveen Soni - 6 years, 6 months ago

Thanks. I have updated the answer to "Only 2, 4".

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Calvin Lin Staff - 6 years, 6 months ago
Ayon Ghosh
Sep 8, 2017

(1) is incorrect because by definition for symmetric matrix A = A T A = A^T that is a i j a_{ij} = = a j i a_{ji} for all i , j i,j .

(2) is correct because by definition for skew symmetric matrix A = A T A = - A^T that is a i j = a j i a_{ij} = -a_{ji} .But now consider the case where i = j = k i = j = k (say).Here a k k = a k k a_{kk} = - a_{kk} implying a k k a_{kk} = = 0 0 for all k k .

for,(3) and (4) see Aman Sharma 's answer it is the perfect way to do it.

P.S. in general any square matrix A A = = 0.5 ( A + A T ) 0.5(A+A^T) + + 0.5 ( A A T ) 0.5 (A-A^T) where A + A T A+A^T is symmetric and A A T A-A^T is skew symmetric.

Anuj Shikarkhane, I think all options are incorrect if you don't specify that: c h a r ( K ) 2 char(K)\neq 2

Math Man
Nov 26, 2014

How to do by instinct

(1) is wrong as a matrix ( 1 ) (1) suffices

then , 3 and 4 can't be both correct, so is is 2 and 4

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