permutation cubics
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B. Odehnal.
Permutation cubics.
To appear in:
ICGG 2020 - Proceedings of the 19th
International Conference on Geometry and Graphics, L.-Y. Cheng (ed.), ICGG 2020,
Advances in Intelligent Systems and Computing, ISBN ..., Sao Paulo, Brazil, August 15-19, 2022,
paper no. 7, 12 pages.
Permuting the trilinear coordinates of a point P yields six (permutation) points that lie on a single conic
which is called the permutation conic.
The six permutation points of P and the six permutation points of the isogonal or isotomic conjugate
of P lie on a cubic curve which shall be called the permutation cubic assciated with P.
To each point in the triangle plane we can assign a permutation cubic and some centers share their
permutation cubic.
Among the many possible permutation cubics only a few rational ones are known and some hundred triangle
centers define a degenerate triangle cubic.
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projective view on the pencil of distance product cubics
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B. Odehnal.
Distance Product Cubics.
KoG 24/24 (2020), 29-40.
Given a triangle in the Euclidean plane, one can ask for the set k of all points
that form a constant product of distances to the side lines of the triangle.
It tourns out that k is an elliptic cubic curve except in one case.
The exceptional case occurs for those points that share the distance product with
the centroid of the base triangle. The centroid is the isolated node on this
particular cubic which is only known cubic, called Bataille acnodal cubic.
The totality of all these distance product cubics forms a pencil of cubics
including the two degenerate cubics: the union of the triangle's side lines and the
three-fold line at infinity.
All cubics share the three real points of inflection which are the three common real
ideal points.
The comparison of the distance product of triangle centers from Kimberling's list
shows that only a small number of centers share their distance product cubics with
other centers.
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The sides of a triangle can be seen at equal angles from the equioptic points of the triangle.
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B. Odehnal: Equioptic points of a triangle.
J. Geom. Graphics 17/1 (2013), 15-24.
The locus of points where two non-concentric circles c1 and c2 are seen
under equal angles is the equioptic circle e.
The equioptic circles of the excircles of a triangle Δ have a common radical axis
r. Therefore the excircles of a triangle share up to
two real points, i.e., the equioptic points of Δ from which the circles can be seen under equal angles.
The line r carries a lot of known triangle centers.
Further we find that any triplet of circles tangent to the sides of Δ
has up to two real equioptic points.
The three radical axis of triplets of circles containing the incircle are concurrent in a new triangle center.
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Chapple's porism causes many circular orbits. |
B. Odehnal: Poristic loci of triangle centers.
J. Geom. Graphics 15/1 (2011), 45-67.
The one-parameter family of triangles with common incircle and circumcircle is called a porisitic system
of triangles. The triangles of a poristic system can be rotated freely about the common incircle.
However, this motion is not a rigid body motion for the sidelengths of the triangle are changing.
Surprisingly many triangle centers associated with the triangles of the poristic family trace circles while
the triangle traverses the poristic family. Other points move on conic sections, some points trace more
complicated curves. We shall describe the orbits of centers and some other points.
Thereby we are able to answer open questions and verify some older results.
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projective version of Gergonne and Nagel points
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B. Odehnal: Generalized Gergonne and Nagel points
Beitr. Algebra Geom. 51/2 (2010), 477-491.
We show that the Gergonne point G of a triangle Δ in the Euclidean plane can in fact be seen from
a more general point of view, i.e., from the viewpoint of projective geometry. So it turns out that there are
up to four Gergonne points associated with Δ. The Gergonne and Nagel point are isotomic conjugates of
each other, and thus we find up to four Nagel points associated with a generic triangle.
We reformulate the problems in a more general setting and illustrate the different appearances of Gergonne points
in different affine geometries. Darboux's cubic can also be found in the more general setting, and finally
a projective version of Feuerbach's circle appears.
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B. Odehnal.
Some triangle centers associated with the
circles tangent to the excircles.
Forum Geometricorum 10 (2010), 35-40.
We study those tritangent circles of the excirlces of a triangle which enclose
exactly one excircle and touch the two others from the outside. It turns out
that these three circles share exactly the Spieker point. Moreover we
show that these circles give rise to some triangles which are in perspective
with the base triangle. The respective perspectors turn out to be new
polynomial triangle centers.
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Publications
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B. Odehnal.
Permutation cubics.
To appear in:
ICGG 2020 - Proceedings of the 19th
International Conference on Geometry and Graphics, L.-Y. Cheng (ed.), ICGG 2020,
Advances in Intelligent Systems and Computing, ISBN ..., Sao Paulo, Brazil, August 15-19, 2022,
paper no. 7, 12 pages.
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B. Odehnal.
Distance Product Cubics.
KoG 24/24 (2020), 29-40.
B. Odehnal.
A one-parameter family of triangle cubics.
In: ICGG 2020 - Proceedings of the 19th International Conference on
Geometry and Graphics,
L.-Y. Cheng (ed.), ICGG 2020, Advances in Intelligent Systems and Computing,
ISBN 978-3-030-63402-5, Sao Paulo, Brazil, August 8-13, 2020, paper no. 4, 38-49.
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B. Odehnal.
Two convergent triangle tunnels.
KoG 22 (2018), 20-28.
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B. Odehnal.
A triad of circles externally tangent to the nine-point circle
and internally tangent to two sides of a triangle.
J. Geom. Graphics 18/1 (2014), 61-71.
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B. Odehnal.
Equioptic points of a triangle.
J. Geom. Graphics 17/1 (2013), 15-24.
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B. Odehnal.
Mixed intersection of Cevians and perspective triangles.
Proc. 15th Internat. Conf. Geometry Graphics, Aug. 1-5, 2012, Montreal/Canada,
article No. 090.
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B. Odehnal.
Poristic loci of triangle centers.
J. Geom. Graphics 15/1 (2011), 45-67.
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B. Odehnal.
Generalized Gergonne and Nagel points
Beitr. Algebra Geom. 51/2 (2010), 477-491.
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B. Odehnal.
Some triangle centers associated with the
circles tangent to the excircles.
Forum Geometricorum 10 (2010), 35-40.
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B. Odehnal.
Three points related to the incenter and excenters of a triangle.
Elem. Math. 61/2 (2006), 74-80.
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