[see also: customary, familiar, usual]
The functions $f_i$ $(i=1,\dots,n)$ have no common zero in $\Omega$.
Then $F$ and $G$ have a factor in common.
It has some basic properties in common with another most important class of functions, namely, the continuous ones.
Take $g_1,\dots,g_n$ without common zero.
Denote by $\theta$ the angle at $x$ that is common to these triangles.
Here we use an inductive procedure very common in geometric model theory.
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