In the case of differential curves, the curvature vector is a normal vector of special interest.
2.
The surface is trapped if both the null expansions ? ?are negative, signifying that the mean curvature vector is timelike and future directed.
3.
The surface is designated trapped if the cross sections of both congruences decrease in area as they exit the surface; and this is apparent in the mean curvature vector, which is:
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