- #1
Chen
- 977
- 1
I need to prove (or disprove) that if A(nxn) is a matrix so that the sum of elements in every row equals 1, then the sum of elements in every row of A-1 also equals 1.
I tried a bunch of examples and they all worked, so I'm assuming this is true and I'm trying to prove it:
We know that:
[tex]A^{-1} = \frac{ajd(A)}{|A|}[/tex]
[tex]\sum_{i=1}^n a_{ki}^{-1} = \sum_{i=1}^n \frac{(adj(A))_{ki}}{|A|} = \sum_{i=1}^n \frac{(-1)^{k+i}M_{ik}}{|A|}[/tex]
But by defintion:
[tex]|A| = \sum_{i=1}^n (-1)^{k+i}a_{ki}M_{ki}[/tex]
So we get:
[tex]\sum_{i=1}^n a_{ki}^{-1} = \frac{\sum_{i=1}^n (-1)^{k+i}M_{ik}}{\sum_{i=1}^n (-1)^{k+i}a_{ki}M_{ki}}[/tex]
And from here I don't know what to do. All that's left is to show that this expression equals 1... somehow. I think I need to use the fact that |A| = 1/|A-1|, no?
Thanks.
I tried a bunch of examples and they all worked, so I'm assuming this is true and I'm trying to prove it:
We know that:
[tex]A^{-1} = \frac{ajd(A)}{|A|}[/tex]
[tex]\sum_{i=1}^n a_{ki}^{-1} = \sum_{i=1}^n \frac{(adj(A))_{ki}}{|A|} = \sum_{i=1}^n \frac{(-1)^{k+i}M_{ik}}{|A|}[/tex]
But by defintion:
[tex]|A| = \sum_{i=1}^n (-1)^{k+i}a_{ki}M_{ki}[/tex]
So we get:
[tex]\sum_{i=1}^n a_{ki}^{-1} = \frac{\sum_{i=1}^n (-1)^{k+i}M_{ik}}{\sum_{i=1}^n (-1)^{k+i}a_{ki}M_{ki}}[/tex]
And from here I don't know what to do. All that's left is to show that this expression equals 1... somehow. I think I need to use the fact that |A| = 1/|A-1|, no?
Thanks.
Last edited: