Lettres grecques en LaTeX

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% Schrödinger equation — \psi, \hbar, \nabla
\[
  i\hbar \frac{\partial \psi(\mathbf{r},t)}{\partial t}
  = \left[ -\frac{\hbar^2}{2m} \nabla^2 + V(\mathbf{r},t) \right] \psi(\mathbf{r},t)
\]

% Maxwell (Gauss's law) — \varepsilon, \rho, \mu
\[
  \nabla \cdot \mathbf{E} = \frac{\rho}{\varepsilon_0}, \qquad
  \nabla \times \mathbf{B} = \mu_0 \mathbf{J}
\]

% Euler-Lagrange — \phi, \partial, \mathcal{L}
\[
  \frac{\partial \mathcal{L}}{\partial \phi}
  - \partial_\mu \frac{\partial \mathcal{L}}{\partial(\partial_\mu \phi)} = 0
\]

% Variant forms side by side
Variants: $\epsilon$ vs $\varepsilon$, \quad
$\phi$ vs $\varphi$, \quad
$\theta$ vs $\vartheta$

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