Why there is the requirement for derivatives no higher than second order in free quantum field equations?
75612
Why are differential equations for fields in physics of order two?
2129
Showing that the derivative operator is not bounded on $L^2 (\mathbb{R})$
87914
Contradictory result for scalar-field propagator from Feynman rules and LSZ formula
157191
Conjugate Variables, Noether's Theorem and QM
131016
Question about Klein-Gordon field Lagrangian
4421
Proof that total derivative is the only function that can be added to Lagrangian without changing the EOM
885
Why is the 2nd derivative written as $\frac{\mathrm d^2y}{\mathrm dx^2}$?
57051
Why does Quantum Field Theory use Lagrangians rather than Hamiltonians?
88660
Time derivative in Schrödinger equation
8844
Higher-order partial derivatives w.r.t. variable raised to some power
117387
Can we safely assume $\Psi(x,t) = \psi(x)e^{-i\omega t}$ always in QM?
38103
A No-Nonsense Introduction to Quantum Field Theory
123911
Why is the derivative of a vector orthogonal to the vector itself?
58122
QM without complex numbers
125774
Derivative of $a^x$ from first principles
107290
proof of product rule of derivatives using first principle?
137152
Free particle Schrodinger equation: propagator