  Wigner gave a well-known proof of Kramers degeneracy, for time reversal
invariant systems containing an odd number of half-integer spin particles. But
Wigner's proof relies on the assumption that the Hamiltonian has an
eigenvector, and thus does not apply to many quantum systems of physical
interest. This note illustrates an algebraic way to talk about Kramers
degeneracy that does not appeal to eigenvectors, and provides a derivation of
Kramers degeneracy in this more general context.
