This man is the alphabetically-former of a namesake technique that uses the angular orbital momentum number as a small parameter to find the mean value of one over r-squared. That technique relates the derivative of total energy to the mean value of the partial derivative of the Hamiltonian (15[1])and is named after this scientist and Hellmann. To expand (15[2])the (15[1])time-evolution operator, (15[1])a sum of constructs named (-5[1])after this scientist (-5[1])can be used to represent (15[1])a (*) Dyson series. (10[1])This scientist’s “trick” (10[2])for single-variable (10[3])integrals (10[4])involves (10[4]-5[1])introducing (10[2])a second variable (10[4])and differentiating (10[1])to simplify the integrand. This scientist’s “diagram” uses arrows (10[1])and waves to (10[1])describe the interactions (10[1])of subatomic particles and draws photons as squiggly lines. For 10 points, name this American theoretical (10[1])physicist. (10[2])■END■