# Square matrices, determinants and consistency

1. Mar 24, 2004

### Asif

This is the first time I'm posting (or rather asking) anything here. I'm a student of elementary linear algebra, therefore plz excuse me if my questions come across as dumb or if I make any mistakes:

I have a question about determinants and whether or not a solution exists, etc. I will be focusing on square matrices only:

If the determinant of a matrix is not equal to zero, then does that mean the matrix has a unique solution?

If the determinant is equal to zero, then either the matrix has infinitely many solutions or no solution, correct?

And if it is a homogeneous system, then the system has infinitely many solutions if the determinant is equal to zero, correct?

Bye.

P.S. Could someone kindly tell me what is meant by singular and non-singular matrices?

2. Mar 24, 2004

### matt grime

One doesn't say a matrix has a solution. Matrices are used to describe sets of linear equations but that is not all they are there for. Second, non-square matrices do not have a determinant.

Non-zero determinant means the matrix is invertible (non-singular), so if you're solving Ax=b, then the answer is given by x=A^{-1}b and the solution is unique.

If the matrix has determinant zero it is singular, and the equations they describe may or may not have solutions. If there is a solution there will be infinitely many of them.

If by homogeneuos you mean Ax=0 then yes there will be an infinite number of solutions as trivially if x satisfies Ax=0 then tx is a solution for any number t, and one of the equivalent statements for determinant zero is that there is *a* non-trivial solution to Ax=0 (ie one where x is not the zero vector).