Invariants of curves in RP2 and R2
Abstract
There is an elegant relation found by Fabricius-Bjerre [Math. Scand 40 (1977) 20--24] among the double tangent lines, crossings, inflections points, and cusps of a singular curve in the plane. We give a new generalization to singular curves in RP2. We note that the quantities in the formula are naturally dual to each other in RP2, and we give a new dual formula.
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