Unitary tridiagonalization inM(4, ⫳)
A question of interest in linear algebra is whether alln xn complex matrices can be unitarily tridiagonalized. The answer for alln ≠ 4 (affirmative or negative) has been known for a while, whereas the casen = 4 seems to have remained open. In this paper we settle then = 4 case in the affirmative. Some machinery from complex algebraic geometry needs to be used.