Function multigrid_vcycle 

pub fn multigrid_vcycle(
    u: &mut Vec<f64>,
    f: &[f64],
    h: f64,
    n_smooth: usize,
    depth: usize,
)

Multigrid V-cycle for a 1-D Poisson problem.

Recursively applies pre-smoothing, coarse-grid correction, and post-smoothing via Gauss-Seidel.

Arguments

• u – Current solution approximation (modified in place).
• f – Right-hand side.
• h – Grid spacing.
• n_smooth – Number of Gauss-Seidel smoothing steps.
• depth – Remaining recursion depth (0 = direct solve).