While self studying Topology from Wayne Patty I was unable to understand this notation on page 218 of the textbook. Subsection is Manifolds.
Consider this statement(which is given as theorem): If X and Y are compact surface, then $\chi (X\mathbin\#Y) = \chi(X)+\chi(Y)- 2$.
Here $\chi(X)$ is Euler characteristic of $X$.
The author then writes:
Assume $X$ and $Y$ are triangulated. Form $X\mathbin\#Y$ by removing the interior of a triangle from each and identifying the edges and vertices of the vertices of the boundaries of the triangles that have been removed.
Is there specific name (mathematical, name of the symbol is $\#$ is clear to me) for the construction $\#$ given in the theorem? How it is constructed is clear to me.
In topology, the operation "#" is called connected sum. Its effect is to join two given manifolds together near a chosen point.