Magnus Gustaf "Gösta" Mittag-Leffler was a Swedish mathematician. His mathematical contributions are connected chiefly with the theory of functions, which today is called complex analysis. He founded the most important mathematical periodical Acta Mathematica and was its editor for 40 years. He took great trouble and procured Sofia Kovalevskaya a position of full professor of mathematics in Stockholm University. Also, Mittag-Leffler was responsible for inducing the Nobel committee to recognize and award Marie Curie as an equal contributor to the discoveries ‘on the radiation phenomena’ along with her husband Pierre Curie.

Complex analysis, traditionally known as the theory of functions of a complex variable, is the branch of mathematical analysis that investigates functions of complex numbers. It is helpful in many branches of mathematics, including algebraic geometry, number theory, analytic combinatorics, and applied mathematics, as well as in physics, including the branches of hydrodynamics, thermodynamics, quantum mechanics, and twistor theory. By extension, use of complex analysis also has applications in engineering fields such as nuclear, aerospace, mechanical and electrical engineering.