Bernstein's problem
In differential geometry, Bernstein's problem is as follows: if the graph of a function on Rn−1 is a minimal surface in Rn, does this imply that the function is linear? This is true for n at most 8, but false for n at least 9. The problem is named for Sergei Natanovich Bernstein who solved the case n = 3 in 1914. Statement

Suppose that f is a function of n − 1 real variables. The graph of f is a surface in Rn, and the condition that this is a minimal surface is that f satisfies the minimal surface equation

Bernstein's problem asks whether an entire function (a function defined throughout Rn−1 ) that solves this equation is necessarily a degree-1 polynomial. History

 proved Bernstein's theorem  that a graph of a real function on R2 that is also a minimal surface in R3 must be a plane. gave a new proof of Bernstein's theorem by deducing it from the fact that there is no non-planar area-minimizing cone in R3. showed that if there is no non-planar area-minimizing cone in Rn−1 then the analogue of Bernstein's theorem is true for graphs in Rn, which in particular implies that it is true in R4. showed there are no non-planar minimizing cones in R4, thus extending Bernstein's theorem to R5. showed there are no non-planar minimizing cones in R7, thus extending Bernstein's theorem to R8. He also showed that the surface defined by

is a locally stable cone in R8, and asked if it is globally area-minimizing. showed that Simons' cone is indeed globally minimizing, and that in Rn for n≥9 there are graphs that are minimal, but not hyperplanes. Combined with the result of Simons, this shows that the analogue of Bernstein's theorem is true in Rn for n≤8, and false in higher dimensions. References

 German translation in

External links
 Encyclopaedia of Mathematics article on the Bernstein theorem

Minimal surfaces
Functions and mappings
Dimension
Theorems in differential geometry