1 | initial version |

Look at the documentation `A.__getitem__?`

for the explanation. In short, `A[0]`

returns the first row of the matrix which is an immutable vector. And `A[0][0]`

returns the first entry of that vector.

If you want the first entry of the matrix directly, then use `A[0,0]`

. In this case, you don't need to work with a copy and you can change the original matrix directly since the matrices are not immutable by default.

```
sage: A = matrix(3,3,0); A
[0 0 0]
[0 0 0]
[0 0 0]
sage: A.is_immutable()
False
sage: A[0,0] = 1; A
[1 0 0]
[0 0 0]
[0 0 0]
sage: A[0]
(1, 0, 0)
sage: A[0].is_immutable()
True
```

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