Oscillation theorems for solutions of hyperbolic equations
Charles S. Kahane · Proceedings of the American Mathematical Society · 1973
It is shown that solutions of hyperbolic equations in cylindrical space time domains which vanish on the lateral boundary of the cylinder must have arbitrarily large zeros in the interior of the cylinder. In case the coefficients of the equation are time independent the solutions will have arbitrarily large zeros on any interior line of the cylinder.