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http://peeterjoot.com/2018/09/23/phy2403h-quantum-field-theory-lecture-4-scalar-action-least-action-principle-euler-lagrange-equations-for-a-field-canonical-quantization-taught-by-prof-erich-poppitz/
PHY2403H Quantum Field Theory. Lecture 4: Scalar action, least action principle, Euler-Lagrange equations for a field, canonical quantization. Taught by Prof. Erich Poppitz