Poisson: −∇·(k∇T) = f in Ω
Weak form: ∫k∇T·∇v dΩ = ∫fv dΩ
Stiffness matrix K_{ij} = ∫k ∇φ_i·∇φ_j dΩ
Linear system: KT = F
Triangle elements: φ_i = 1 on node i, 0 on others
Piecewise-linear → 1st-order FEM
Sparsity: ~6 entries per row (local support)
Solved via Gauss-Seidel iteration