This is the first word in the name of a hybrid quantum-classical algorithm introduced by Peruzzo et al. with the most promising near-term potential for speeding up computational chemistry calculations. That “quantum eigensolver” described by this word takes advantage of the fact that the expectation of a Hermitian matrix is bounded below by its lowest eigenvalue. (10[1])An approximation method named for this word minimizes the energy of a trial wavefunction with respect (10[1])to a parameter. The Beltrami identity is key to a subfield of mathematical analysis whose name includes this plural noun, which uses the Euler–Lagrange equation to find the extrema of functionals like the action (10[1])in Lagrangian mechanics. The brachistochrone problem motivated the development of, for 10 points, the “calculus of” what word, meaning small changes to functions? ■END■