Alexandr Lyapunov was a prominent Russian mathematician and physicist, born on June 6, 1857. He is best known for his work in the field of stability theory, particularly in the study of dynamical systems. His most significant contribution is the Lyapunov stability criterion, which helps determine the stability of equilibrium points in differential equations. Lyapunov also made important advancements in probability theory and applied mathematics. He developed the Lyapunov central limit theorem, which is a fundamental result in statistics. His work laid the groundwork for many modern mathematical theories and continues to influence various scientific disciplines today.