What is the amalg symbol?

Question

I saw this formula: $A \amalg B$, where $A$ and $B$ are sets. I searched for the name of the symbol, "amalg", but haven't found a definition. What is the meaning of this symbol?

Answer 1

As Peter Franek said, in the comments this typically means the disjoint union of $A$ and $B$. I think it is a bit more common (in modern writing) to see $A \sqcup B$. More generally this could also be talking about coproducts of the objects $A$ and $B$, which in the category of sets is disjoint union. For example if your sets are groups then the coproduct of groups is different from disjoint union, it is also known as the free product in this case, although free product of groups is normally denoted $A * B$.

Answer 2

I am reading some notes from a hungarian friend. It seems to me like this means "independent" random variables in probability.

I was wondering if this is common notation or if it's common only in Hungary.

Answer 3

I think the name "amalg" may come from the terminology free product with amalgamation, which occurs in algebraic topology, for instance in Van Kampen's theorem.