Maths-
General
Easy

Question

Assertion : There are only finitely many 2 × 2 matrices which commute with the matrix
open square brackets table attributes columnalign center center columnspacing 1em end attributes row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square brackets
Reason : If A is non-singular then it commutes with I, adj A and A–1

  1. If both (A) and (R) are true, and (R) is the correct explanation of (A).    
  2. If both (A) and (R) are true but (R) is not the correct explanation of (A).    
  3. If (A) is true but (R) is false.    
  4. If (A) is false but (R) is true.    

The correct answer is: If (A) is false but (R) is true.


    The reason R is true since
    AI = IA, AA–1 = A–1A = I, A|adj A| = |adj. A|A
    But a matrix can commute with general order matrices which may be infinite in number.
    Let B = open square brackets table attributes columnalign left left columnspacing 1em end attributes row cell a      b end cell row cell c      d end cell end table close square brackets be a matrix which commute with A then AB = BA
    table attributes columnalign right left right left right left right left right left right left columnspacing 0em 2em 0em 2em 0em 2em 0em 2em 0em 2em 0em end attributes row cell not stretchy rightwards double arrow open square brackets table attributes columnalign center center columnspacing 1em end attributes row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square brackets open square brackets table attributes columnalign left left columnspacing 1em end attributes row a b row c d end table close square brackets equals open square brackets table attributes columnalign left left columnspacing 1em end attributes row a b row c d end table close square brackets open square brackets table attributes columnalign center center columnspacing 1em end attributes row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square brackets end cell row cell equals open square brackets table attributes columnalign center center columnspacing 1em end attributes row cell a plus 2 c end cell cell b plus 2 d end cell row cell negative a minus c end cell cell negative b minus d end cell end table close square brackets equals open square brackets table attributes columnalign left left columnspacing 1em end attributes row cell a minus b end cell cell 2 a minus b end cell row cell c minus d end cell cell 2 c minus d end cell end table close square brackets end cell row cell not stretchy rightwards double arrow a plus 2 c equals a minus b comma b plus 2 d equals 2 a minus b comma negative a minus c end cell row cell equals c minus d comma negative b minus d equals 2 c minus d end cell end table
    The above four relations are equivalent to only two independent relations
    a – d = b, b + 2c = 0
    If d = lambda, then a = b + lambda = –2c + lambda
    Thus,  open square brackets table attributes columnalign center center columnspacing 1em end attributes row cell lambda minus 2 c end cell cell negative 2 c end cell row c lambda end table close square brackets are all possible 2 × 2 matrices which commute with given matrix A = open square brackets table attributes columnalign center center columnspacing 1em end attributes row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square brackets
    lambda and c being any arbitrary complex numbers. Thus assertion is therefore false.

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