This is ["The Wall of Darkness"](http://www.isfdb.org/cgi-bin/title.cgi?62791) by Arthur C. Clarke, as previously discussed in [this question.](https://scifi.stackexchange.com/questions/63411/what-is-the-geometry-of-the-universe-in-the-wall-of-darkness-by-arthur-c-clark)

A [summary](http://kasmana.people.cofc.edu/MATHFICT/mfview.php?callnumber=mf319) from Alex Kasman's Mathematical Fiction site:

> In a universe consisting of one star and one planet, there is a mysterious impenetrable wall surrounding the entire planet in the deep freezing southlands. Two men, one with money, the other with building skills, engage in a long-term program to scale the wall and find out what's on the other side. The answer turns out to be mathematical. And rather upsetting.