Why is it that Spanish cinema has produced all the meagre amount of win in this festival, by letting slip a couple of films (this and Time Crimes) of new productions that aren't social(ist) realist? That's a trickier question than most of the ones posed in this film to the four mathematicians trapped in a shrinking room -- those are on the level of the standard Microsoft interview puzzle of yore (including the old chestnut about the box with the three switches and a lightbulb) -- another Diamond Dogs, this is not. Another tricky question is "why did it start to get dark so early in late July?" -- sunset on the 25th of July in Gibraltar would be 21:32 local time, with half an hour of twilight, or 15 minutes earlier for Barcelona. And why were they so overdressed for summer weather?
A limited cast, a sealed room, an implausible level of skeletons emerging from closets later, we are served a somewhat anti-climactic conclusion after the initial build up. Personally, I was disappointed in all the puzzle-solving that the four key members of the cast were distressingly two-dimensional in their thinking.
Still, it was interesting to see Goldbach's Conjecture used as a motive for murder, presumably because it is a puzzle that can be explained, unlike the more useful, but equally unsolved, Riemann Hypothesis about the zeroes of the ζ-function.