\documentclass[12pt]{article} \begin{document} \large \[ f(x,y) = \sum_{j=0}^{M-1} f_j \phi_j(x, y) \] \[ \min_f \sum_{i=0}^{N-1} (f(x_i, y_i) - z_i)^2 \ + \ \alpha \int_\Omega (\nabla f)^2 \, dx dy \] \end{document}