TeX source:

-\frac{\partial^{2}\psi_{1}(x)}{\partial x^{2}}\cdot\frac{1}{\psi_{1}(x)}- \frac{\partial^{2}\psi_{2}(y)}{\partial y^{2}}\cdot\frac{1}{\psi_{2}(y)}- \frac{\partial^{2}\psi_{3}(z)}{\partial z^{2}}\cdot\frac{1}{\psi_{3}(z)}=k^{2}