[Math] Is the direct image of a constant sheaf a constant sheaf

Question

Is the direct image of a constant sheaf a constant sheaf? I'm not an expert on sheaf theory and can't find this anywhere

Answer 1

Notation: $f:X \to Y$ is the map we're pushing forward along, and $F$ is our sheaf on $X$. In general the stalks of $f_*F$ at different points will not be isomorphic. For instance if $f$ misses the point $y \in Y$ and your space is sufficiently separated then the stalk of $f_*F$ at $y$ will be 0 while it will be nonzero for points in the image.

An extreme case is when the map has image a point. Then you get a skyscraper sheaf, which is very far from constant on most spaces and most points (Note: if you're hitting the generic point of $Y$ then the direct image will in fact be constant).

Edit: Another extreme case is when $X$ is a large discrete space. Then one can get direct image sheaves where no stalk is isomorphic to any other stalk. For instance this happens if all the fibers of $f$ have different cardinalities. I think you would usually need the axiom of choice to even define such a map.