What are some unsolved problems in math which seem easy at first glance?

Here's a good one that's slightly less well known: if 2x and 3x are both integers, must x be an integer as well?

(This is known to be the case if you also assume that 5x is an integer, but that's an extremely difficult theorem.)

Oh, and here's another: can you color the points of the plane in 4 colors so that no two points 1m apart have the same color? How about 5 colors? 6?

(With 3, you definitely can't (cool puzzle). With 7, you definitely can (slightly less cool puzzle). Nobody knows what the minimum number of colors is).


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