While I'm reading some research papers on convexity and preinvex functions (which is a generalization of convexity) I noticed that sometimes authors assume that the domain of a convex (preinvex) function is a subset of $\mathbb{R}$, and sometimes it's subset of the positive real line, i.e., $K\subset [0,\infty)$. Is there a specific reason for this condition?
Among many articles, we have the following:
https://sigma.yildiz.edu.tr/storage/upload/pdfs/1635932447-en.pdf
and