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We add a little bit to On 'fair bisectors' of planar convex regions and Bisectors and partitioning lines for convex regions defined with respect to the moment of inertia

Definitions: Given a planar convex region C (could be smooth or polygonal), an area bisector of C is any line that partitions C into 2 pieces of equal area. A perimeter bisector is a line that partitions C into 2 pieces of equal perimeter. Obviously, thru every point on the boundary of C we can draw an area bisector and a perimeter bisector.

Question: Are the following claims easy to prove/counter?

  • A planar convex region is centrally symmetric if and only if its area bisectors are all concurrent.
  • A planar convex region is centrally symmetric if and only if its perimeter bisectors are all concurrent.

Note: Higher dimensional analogs of these claims are easy to state.

Nandakumar R
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  • A planar convex region is centrally symmetric if and only if its area bisectors are all concurrent.

Yes. Let all area bisectors of our region $K$ pass through a point $O$. Note that any line $\ell$ through $O$ is an area bisector of $K$: otherwise there exists a parallel area bisector (by continuity), and it does not pass through $O$. Assume that $K$ is not $O$-symmetric. It yields that some chord $AB$ through $O$ is not bisected by $O$. Move it slightly to get a chord $A_1B_1$ ($A_1$ close to $A$). The curvilinear triangles $AOA_1$ and $BOB_1$ have unequal area, thus at least one of chords $AB$, $A_1B_1$ is not an area bisector.

Fedor Petrov
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  • Thanks very much. The same argument seems to prove the other claim as well - A_1B_1 is got by slightly rotating AB about O. Arcs AA_1 and BB_1 will not be of same length if O is not a center of symmetry so both chords AB and A_1B_1 wont be perimeter bisectors. – Nandakumar R Jun 12 '23 at 18:03
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    I am afraid that for perimeters things are different. The derivative involves the angle between the chord and the tangent – Fedor Petrov Jun 12 '23 at 18:51
  • Thanks for the correction. Shall mark this question as solved and look forward to a settlement to the perimeter issue. – Nandakumar R Jun 13 '23 at 10:07