So, I noticed something weird while using a library math.js: there is a function called
usolve, that is supposed to find some solution of the equation $U\cdot x = b$, where $U$ is an upper-triangular matrix. This function requires $U$ to be square matrix.
I did a bit of googling and it looks like other math-tools such as scipy or matlab also require the matrix to be square for solving such an equation.
Question: why is it so?
For me it looks like you can just do some fixing by adding zeros here and there and make a square matrix out of $U$, so this requirement
is just there to annoy people serves no purpose.
UPD: I am going to make an update to this question explaining in details how backsub-method would work for an upper-triangular matrix of any size... Any time now.