Matrix Flipping

Algebra Level 3

Let { A i } \displaystyle \{ A_i \} be a set of invertible m × m m \times m matrices with 1 i n 1 \le i \le n . If A 1 A^{-1} denotes the inverse of a matrix A A , is the following statement true for all n ? n?

( A 1 A 2 A n 1 A n ) 1 = A n 1 A n 1 1 A 2 1 A 1 1 . \left( A_1 \cdot A_2 \cdots A_{n-1} \cdot A_n \right)^{-1} = A_n^{-1} \cdot A_{n-1}^{-1} \cdots A_2^{-1} \cdot A_1^{-1}.

No Depends on n n Yes

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1 solution

Akeel Howell
Jul 12, 2018

For any pair of invertible matrices A A and B B which can be multiplied, the inverse of the matrix A B AB is ( A B ) 1 = B 1 A 1 \left( AB \right)^{-1} = B^{-1}A^{-1} .

Now, we can proceed by induction on n n for the inverse of the product of n n invertible m × m m \times m matrices: ( A 1 A 2 A n 1 A n ) 1 = ( ( A 1 A 2 A n 1 ) A n ) 1 = A n 1 ( A 1 A 2 A n 2 A n 1 ) 1 = = A n 1 A n 1 1 A 2 1 A 1 1 . \left( A_1 \cdot A_2 \cdots A_{n-1} \cdot A_n \right)^{-1} = \left( \left( A_1 \cdot A_2 \cdots A_{n-1} \right) \cdot A_n \right)^{-1} \\ = A_n^{-1} \cdot \left( A_1 \cdot A_2 \cdots A_{n-2} \cdot A_{n-1} \right)^{-1} = \cdots = A_n^{-1} \cdot A_{n-1}^{-1} \cdots A_2^{-1} \cdot A_1^{-1}.

Hence, the statement is true for all n n .

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