If f\colon M\to\R^3 is a tight immersion, then
M can be decomposed into two regions, M^+ and M^- where
- K\ge 0 on M^+ and
K\le 0 on M^-
- f embeds M^+ onto
the complement in \dHf of a finite number of
disjoint planar convex closed disks
D_1,\ldots,D_k in \dHf
- for each i,
f^{-1}(\partial D_i) is a non-trivial 1-cycle in M
(these are called top cycles)
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