Vladimir Igorevich Arnold (alternative spelling Arnol'd, Russian: Влади́мир И́горевич Арно́льд , 12 June 1937 – 3 June 2010) was a Soviet and Russian mathematician.
 While he is best known for the Kolmogorov–Arnold–Moser theorem regarding the stability of integrable systems, he made important contributions in several areas including dynamical systems theory, catastrophe theory, topology, algebraic geometry, symplectic geometry, differential equations, classical mechanics, hydrodynamics and singularity theory, including posing the ADE classification problem, since his first main result—the solution of Hilbert's thirteenth problem in 1957 at the age of 19.
In real algebraic geometry, Gudkov's conjecture, also called Gudkov’s congruence, (named after D. A. Gudkov) was a conjecture, and is now a theorem, which states that "a M-curve* of even degree "2d" obeys "p" – "n" ≡ "d" (mod 8)", where "p" is the number of positive ovals and "n" the number of negative ovals of the M-curve.
 It was proved by the combined works of Vladimir Arnold and Vladimir Rokhlin.
Evgeny "Eugen" Evgenievich Slutsky ( ; ( Евге́ний Евге́ньевич Слу́цкий) 7 April [O.S. 19 April] 1880 – 10 March 1948) was a Russian/Soviet mathematical statistician, economist and political economist.
Smale's problems are a list of eighteen unsolved problems in mathematics that was proposed by Steve Smale in 1998, republished in 1999.
 Smale composed this list in reply to a request from Vladimir Arnold, then vice-president of the International Mathematical Union, who asked several mathematicians to propose a list of problems for the 21st century.
 Arnold's inspiration came from the list of Hilbert's problems that had been published at the beginning of the 20th century.
In mathematics, Arnold's cat map is a chaotic map from the torus into itself, named after Vladimir Arnold, who demonstrated its effects in the 1960s using an image of a cat, hence the name.
The Slutsky equation (or Slutsky identity) in economics, named after Eugen Slutsky, relates changes in Marshallian (uncompensated) demand to changes in Hicksian (compensated) demand, which is known as such since it compensates to maintain a fixed level of utility.
 The equation demonstrates that the change in the demand for a good, caused by a price change, is the result of two effects:
In applied mathematics, Arnold diffusion is the phenomenon of instability of integrable Hamiltonian systems.
 The phenomenon is named after Vladimir Arnold who was the first to publish a result in the field in 1964.
 More precisely, Arnold diffusion refers to results asserting the existence of solutions to nearly integrable Hamiltonian systems that exhibit a significant change in the action variables.
Gabriella Pinzari is an Italian mathematician.
 Her research on the n -body problem has been described as "the most natural way to apply" the Kolmogorov–Arnold–Moser theorem to the problem.
 The original work of Vladimir Arnold on this theorem attempted to use it to show the stability of the solar system or similar systems of planetary orbits, but this worked only for the three-body problem because of a degeneracy in Arnold's mathematical framework.
 Pinzari showed how to eliminate this problem, and extend the solution to larger numbers of bodies, by developing "a rotation-invariant version of the KAM theory".
In mathematics, Arnold's spectral sequence (also spelled Arnol'd) is a spectral sequence used in singularity theory and normal form theory as an efficient computational tool for reducing a function to canonical form near critical points.
 It was introduced by Vladimir Arnold in 1975.
In dynamical systems theory, the Liouville–Arnold theorem states that if, in a Hamiltonian dynamical system with "n" degrees of freedom, there are also known "n" first integrals of motion that are independent and in involution, then there exists a canonical transformation to action-angle coordinates in which the transformed Hamiltonian is dependent only upon the action coordinates and the angle coordinates evolve linearly in time.
 Thus the equations of motion for the system can be solved in quadratures if the canonical transform is explicitly known.
 The theorem is named after Joseph Liouville and Vladimir Arnold.

Question: Whose contributions to math came later, Eugen Slutsky or Vladimir Arnold?
Answer: Vladimir Igorevich Arnold