I am old and after 56 years of professional experience in research (in the area of numerical simulation), I still have a lot of fun with mathematics (almost with the problems I still did not get the solutions of).
My major concern is not one calculation. In simulation (dynamic processes, oil and gas reservoirs,...) most calculations must be repeated zillions of times. So, numerical efficiency is my main concern.
I can be contacted by e-mail at firstname.lastname@example.org