Pure states, mixtures, ensemble averages, cyclic trace, and the von Neumann equation — all the core properties of the density operator worked out explicitly.

Deriving the electron neutrino survival probability from scratch using two bases, a rotation angle, and time evolution — plus why oscillations prove neutrinos have mass.

We decompose L·S using ladder operators and write the spin-orbit Hamiltonian explicitly as a 2×2 matrix in the Sz eigenbasis.

Given a general spin-1/2 state, we calculate the probabilities of measuring positive spin along y and z, and verify with known eigenstates.

Deriving the explicit matrix form of the time-reversal operator for a spin-1/2 particle by expanding the exponential, and proving the famous result Θ² = −1 for fermions.

Using angular momentum conservation to constrain the final state of a particle decay, enumerating possible states, determining parity, and extracting spin measurement probabilities from Clebsch-Gordan coefficients.

Working through a discrete rotation group, its subgroups, eigenvalues of symmetry operators, non-commutativity of rotations and reflections, and using symmetry to identify vanishing matrix elements.