Later in his career he worked in optimal control theory. His maximum principle is fundamental to the modern theory of optimization. He also introduced there the idea of a bang-bang principle, to describe situations where either the maximum 'steer' should be applied to a system, or none.
Pontryagin was a controversial personality. Although he had many Jews among his friends and supported them in his green years, he was accused of anti-Semitism in his mature years. For example he attacked Nathan Jacobson while both men were vice-presidents of the International Mathematical Union. He rejected those charges in an article published in Science in 1979. He also participated in a few notorious political campaigns in the Soviet Union, in particular, in the so-called Luzin affair.
Data Assimilation as a Problem in Optimal Tracking: Application of Pontryagin's Minimum Principle to Atmospheric Science
Apr 01, 2013; ABSTRACTA data assimilation strategy based on feedback control has been developed for the geophysical sciences- a strategy that...