Left eigenvalues of $2\times 2$ symplectic matrices
  We obtain a complete characterization of the $2\times 2$ symplectic matrices having an infinite number of left eigenvalues. Previously, we give a new proof of a result from Huang and So about the number of eigenvalues of a quaternionic matrix. This is achieved by applying an algorithm for the resolution of equations due to De Leo et al. 
E. Mac\'ias-Virg\'os, M.J. Pereira-S\'aez