"Eigenvalues"@en . . "Sturm-Liouville operators"@en . . "e1f7fb9e8152b0ff8d609c25044f0d7c" . "Extremal problems"@en . "Calculus of variations"@en . "Control theory"@en . "Eigenfunctions"@en . "Thibault Liard" . "Ingham's inequality"@en . . "Liard" . "Thibault" . "Characteristics method"@en . "Sturm-Liouville problems"@en . "Wave equation"@en .