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mathematical term for a rectangular array of numbers.
A matrix is said to be an mxn (read "m by n") matrix if it has m rows and n columns. Ie. A 4x4 matrix looks like this:

| 1 2 3 4 |
| 2 2 2 2 |
| 3 5 3 3 |
| 4 4 4 4 |


Each entry in the matrix is denoted as a_i,j where i is the row number and j is the column number
Ie. in the example 4x4 matrix the a_3,2 entry is equal to 5