Given two smooth projective schemes $X$ and $Y$ over some algebraically closed fields $k$, one has the product $X\times Y$ with the projections $\pi\_X$ and $\pi\_Y$.


Now i have a coherent sheaf $M$ on $X$ and a coherent sheaf $N$ on $Y$, and i have locally free resolutions $M\_{\\*}\rightarrow M$ of length m and $N\_{\\*}\rightarrow N$ of length n for cohomological computations. But i want to work with $\pi\_X^{\\*}M$ and $\pi\_Y^{\\*}N$ on $X\times Y$. The easiest way would be if i could pullback the resolutions too, so i don't have to get new resolutions on the product. But as the pullback is just right exact in general, one needs flatness of the projections for this.


So i tried to see if the projectiosn are flat. But for example $\pi\_X$ is given by the base change of $Y\rightarrow Spec(k)$ by $X\rightarrow Spec(k)$. Since $Y\rightarrow Spec(k)$ is flat, and flatness is preserved under base change one has that $\pi\_X$ is flat. Could it be that easy?



Flatness of the canonical projections
Yes. If $f: X\to S$ is a flat morphism and $S'\to S$ is an arbitrary morphism, then the projection $f\_{S'}: X\times\_S S'\to S'$ is flat. A reference is EGA IV.2.1.4.


