# What is this expression?

I saw this nice relief on the University of Warsaw's library building, but I'm left wondering what the line beneath $\pi$ and above Collatz is saying. I'm not familiar with the arrow notation.

Can anyone identify it or give a reference for it?

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That is the long exact sequence of homology groups of a pair $(X,A)$ of topological spaces. –  Stefan Hamcke Jul 28 '13 at 15:54
Looks like Mayer-Vietoris en.wikipedia.org/wiki/Mayer%E2%80%93Vietoris_sequence to me –  BlackAdder Jul 28 '13 at 15:55