Noether's Second Theorem: Mathematics, Functional, Differential Equation, Lagrangian

Oscillation (Differential Equation): Mathematics, Ordinary Differential Equation, Kneser Theorem, Sturm Picone Comparison Theorem

Fuchs's Theorem: Mathematics, Differential Equation, Frobenius Series

Carathéodory's Existence Theorem: Mathematics, Ordinary Differential Equation, Peano Theorem

Inverse Problem for Lagrangian Mechanics: Mathematics, Ordinary Differential Equation, Euler Lagrange