I = baseLocusOfMap(h)
I = baseLocusOfMap(phi)
This defines the locus where a given map of projective varieties is not defined. If the option SaturateOutput is set to false, the output will not be saturated. The default value is true. Consider the following rational map from $P^2$ to $P^1$.
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Observe it is not defined at the point [0:0:1], which is exactly what one expects. However, we can restrict the map to a curve in $P^2$ and then it will be defined everywhere.
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Let us next consider the quadratic Cremona transformation.
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The base locus is exactly the three points one expects.
The object baseLocusOfMap is a method function with options.