I was wondering what was formally correct, when resolving an integral by substitution, use as a new variable $t$ or $u$? I noticed that here in Italy we use $t$, while I saw that abroad (e.g USA) use $u$. When was this variable method of integration designed?
I am aware that I can use any variable and that the result would be the same but I'm curious to know originally what was used by Newton, Leibniz, etc.
I hope someone knows how to answer, it is an unresolved doubt that I have for some time.