Question: Aleksandr Lyapunov and Israel Gelfand, share which mutual occupation?

Context paragraphs:
Lyapunov–Malkin theorem: The Lyapunov–Malkin theorem (named for Aleksandr Lyapunov and ) is a mathematical theorem detailing nonlinear stability of systems.

Israel Gelfand: Israel Moiseevich Gelfand, also written Israïl Moyseyovich Gel'fand, or Izrail M. Gelfand (Yiddish: ישראל געלפֿאַנד‎ , Russian: Изра́иль Моисе́евич Гельфа́нд ; 2 September [O.S. 20 August] 1913 – 5 October 2009) was a prominent Soviet mathematician.  He made significant contributions to many branches of mathematics, including group theory, representation theory and functional analysis.  The recipient of many awards, including the Order of Lenin and the Wolf Prize, he was a Fellow of the Royal Society and professor at Moscow State University and, after immigrating to the United States shortly before his 76th birthday, at Rutgers University.

Liouville–Bratu–Gelfand equation: In mathematics, Liouville–Bratu–Gelfand equation or Lioville's equation is a non-linear eigenvalue Poisson equation, named after the mathematicians Joseph Liouville, G. Bratu and Israel Gelfand.  The equation reads

Lyapunov vector: In applied mathematics and dynamical system theory, Lyapunov vectors, named after Aleksandr Lyapunov, describe characteristic expanding and contracting directions of a dynamical system.  They have been used in predictability analysis and as initial perturbations for ensemble forecasting in numerical weather prediction.  In modern practice they are often replaced by bred vectors for this purpose.

Gelfand–Mazur theorem: In operator theory, the Gelfand–Mazur theorem is a theorem named after Israel Gelfand and Stanisław Mazur which states that a Banach algebra with unit over the complex numbers in which every nonzero element is invertible is isometrically isomorphic to the complex numbers, i. e., the only complex Banach algebra that is a division algebra is the complex numbers C.

Lyapunov function: In the theory of ordinary differential equations (ODEs), Lyapunov functions are scalar functions that may be used to prove the stability of an equilibrium of an ODE.  Named after the Russian mathematician Aleksandr Mikhailovich Lyapunov, Lyapunov functions (also called the Lyapunov’s second method for stability) are important to stability theory of dynamical systems and control theory.  A similar concept appears in the theory of general state space Markov chains, usually under the name Foster–Lyapunov functions.

Aleksandr Lyapunov: Aleksandr Mikhailovich Lyapunov (Russian: Алекса́ндр Миха́йлович Ляпуно́в , ] ; June 6 [O.S. May 25] 1857 – November 3, 1918) was a Russian mathematician, mechanician and physicist.  His surname is sometimes romanized as Ljapunov, Liapunov, Liapounoff or Ljapunow.  He was the son of astronomer Mikhail Lyapunov and the brother of pianist and composer Sergei Lyapunov.

Lyapunov–Schmidt reduction: In mathematics, the Lyapunov–Schmidt reduction or Lyapunov–Schmidt construction is used to study solutions to nonlinear equations in the case when the implicit function theorem does not work.  It permits the reduction of infinite-dimensional equations in Banach spaces to finite-dimensional equations.  It is named after Aleksandr Lyapunov and Erhard Schmidt.

Lyapunov (crater): Lyapunov is a lunar crater named for Aleksandr Lyapunov that is located along the east-northeastern limb of the Moon, and is viewed from the side by observers on Earth.  The eastern rim of this crater just falls into the section of the Moon called the far side, and visibility of this formation is affected by libration.

Lyapunov time: In mathematics, the Lyapunov time is the characteristic timescale on which a dynamical system is chaotic.  It is named after the Russian mathematician Aleksandr Lyapunov.  It is defined as the inverse of a system's largest Lyapunov exponent.

Supporting paragraphs: Aleksandr Lyapunov Israel Gelfand
Answer: mathematician