To use the Defined by Function option, first define a function f (i, j) of two variables. Then use Defined by Function to create the m×n matrix with (i, j) entry equal to f (i, j) for 1≤i≤m and 1≤j≤n.
You can use Fill Matrix to create a general matrix with entries such as ai, j.
Note the comma between subscripts. Without the comma, the subscript ij would be interpreted as a product! You can use the following trick to create a general matrix up to 9×9 with no commas in the subscripts.