An intuitive explaination is imagine a nearly diagonal matrix where the values along the diagonal are much larger than values on the off diagonal. We know the

An intuitive explaination is imagine a nearly diagonal matrix where the values along the diagonal are much larger than values on the off diagonal. We know the eigenvalues of a diagonal matrix is simply the values on the diagonal, so for nearly diagonal matrices you can be pretty sure that the true eigenvalues are going to be pretty close to those diagonal entries, but a natural question to ask is how far we'd deviate from those diagonal entries. The answer to the above question is Gerschgorin disks and it's closely related cousin Brauer's Oval of Cassini. For matrices with real eigenvalues it's moreso along the real number line, only for cases where the eigenvalues are imaginary do we imagine disks.