I am doing some homework currently and I came across a symbol I don't recognise. The question itself is about the Euler characteristic of a space and various statements to prove. I don't want help with the question I just want to know what the symbol stands for.
Let $S_1,S_2$ be two surfaces. Prove that $\chi(S_1 \# S_2 )=\chi(S_1)+\chi(S_2)-2$
My question is, what does the $\#$ in $S_1\# S_2$ mean?
As mentioned in the comments, if $M_1$, $M_2$ are connected manifolds of dimension $n$, then $M_1\# M_2$ denotes their connected sum, which is again a connected manifold of dimension $n$.