Question: Information:  - Sir Alexander Oppenheim , OBE FRSE PMN ( 4 February 1903 -- 13 December 1997 ) was a British mathematician . In mathematics , his most notable contribution is his Oppenheim conjecture .  - Ergodic theory (Ancient Greek: "ergon" work, "hodos" way) is a branch of mathematics that studies dynamical systems with an invariant measure and related problems. Its initial development was motivated by problems of statistical physics.  - Mathematics (from Greek  "máthma", knowledge, study, learning) is the study of topics such as quantity (numbers), structure, space, and change. There is a range of views among mathematicians and philosophers as to the exact scope and definition of mathematics.  - In number theory, the field of Diophantine approximation deals with the approximation of real numbers by rational numbers. It is named after Diophantus of Alexandria.  - A mathematical problem is a problem that is amenable to being represented, analyzed, and possibly solved, with the methods of mathematics. This can be a real-world problem, such as computing the orbits of the planets in the solar system, or a problem of a more abstract nature, such as Hilbert's problems.<br> It can also be a problem referring to the nature of mathematics itself, such as Russell's Paradox.  - A mathematician is someone who uses an extensive knowledge of mathematics in his/her work, typically to solve mathematical problems.  - In Diophantine approximation, the Oppenheim conjecture concerns representations of numbers by real quadratic forms in several variables. It was formulated in 1929 by Alexander Oppenheim and later the conjectured property was further strengthened by Davenport and Oppenheim. Initial research on this problem took the number "n" of variables to be large, and applied a version of the Hardy-Littlewood circle method. The definitive work of Margulis, settling the conjecture in the affirmative, used methods arising from ergodic theory and the study of discrete subgroups of semisimple Lie groups.  - In mathematics, a quadratic form is a homogeneous polynomial of degree two in a number of variables. For example, is a quadratic form in the variables "x" and "y".    Given the information above, choose from the list below the object entity that exhibits the relation 'field of work' with the subject 'alexander oppenheim'.  Choices: - ergodic theory  - mathematician  - mathematics  - nature  - number theory  - physics  - research
Answer:
number theory