[dchapp]

It's not that eigenvalues per se are useless--they're plainly not--but that no one finds eigenvalues in practice by computing the characteristic polynomial and solving for its roots. Unfortunately, that computation is often found in HW and exams in US undergraduate linear algebra courses.

[fa]

This is especially tragic because matrix factorization algorithms are so deep and interesting, theory and programming-wise! LU, Cholesky, QR, eigendecomposition, SVD, mmmm. Round-off error tolerance, convergence criteria, stability, yum.

Characteristic qualities and root finding: bleh.

[east2west]

What is interesting is obviously personal, and I know you don't mean to denigrate some topics in general, but I want to caution people that characteristic equations and root finding are complex and far from useless. Numerical computation of eigen decomposition starts from finding the roots of the characteristic equations (in altered form), the eigen values, without which there is no computation of SVD. How the behaviors of roots change as coefficients vary are fundamental in control engineering. Newton's method holds up half of numerical optimization. Undergraduates don't have to learn them because others have worked out the details and implemented them in software.

By the way, Cramer's rule is useless for numerical computation, but it is immensely useful in theoretical work. It belongs to the vast body of work dealing with determinants before the rise of linear algebra. Determinant is the only obvious connection to algebra left in an undergraduate's linear algebra course, so I can understand people are turned off by it.

[dchapp]

Not to mention that when your matrix is 5x5 or more, there aren't even general solutions for roots if for some reason you're still insisting on the matrix->polynomial->eigenvalues route.

[Chinjut]

Sure there are (in many reasonable senses). Polynomial root extraction just isn't expressible in terms of addition, subtraction, multiplication, division, and nth roots alone. But that's ok; there's nothing magic about that particular set of operations, so as to make it the end-all, be-all.

[j2kun]

People also don't only solve for eigenvalues computationally. Knowing about all the perspectives of eigenvalues helps. I agree the computations are dumb, but if you propose an intro linear algebra class with no computations to soak up test scores you will get far more protests.