Is $-\infty -i $ still equal to $ -\infty$? In the text below, I kind of saw something like $icosh(\infty+iπ/2)-i=-\infty$? As far as I know, $cosh(\infty+iπ/2)=i\infty$, thus $i(i\infty)=-\infty$. Now, is $-\infty-i$ equal to only -$\infty$ according to the text?
How is the substitution I putted in blue working? No matter what I do, I can not get the same result as in the text.
