Robert Ferréol
24 articles published in Tangente
Math for everyoneN°84Nov 18, 2022Tangent or asymptote?
Confusing "tangent" and "asymptote" is a very common mistake: what student has never made it? Yet the distinction between the two notions is perfectly clear. An exploration of certain constructions in projective geometry will nonetheless force us to call our certainties into question.
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Math for everyoneN°81Feb 16, 2022Surprising shapes in space
As in the plane, many balls associated with different norms can be defined in space. They include polyhedra, some regular and others more surprising. You can thus make a delightful festive collection of baubles to match your next Christmas tree!
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Math for everyoneN°81Feb 15, 2022Balls in the plane
A spherical ball is the set of points whose distance from a center is less than a constant. Mathematicians, always seeking to generalize, accept this definition of a ball… in any space equipped with any kind of "distance."
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Math for everyoneN°81Feb 14, 2022The point farthest from France's borders
Where is the "true" center of France? The question sometimes crops up among fans of recreational mathematics and divides enthusiasts of geographical curiosities. Depending on the criterion used to define this center, it may lie in the Cher, the Indre or… the Finistère!
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Math for everyoneN°80Nov 18, 2021The Klein group and its many guises
When we first start working with groups, we patiently draw up the tables for those with only a few elements. A one-element group consists solely of the identity element and is therefore unique. Similarly, groups with two or three elements are unambiguously determined. The surprises begin with four elements...
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Math for everyoneN°78May 18, 2021Triangles with an angle trisector that is a median
A geometric question about trisecting an angle can lead to some astonishing discoveries! Along the way, we encounter triangles, angle trisectors that are also medians, the non-constructible 20° angle—and curves not yet catalogued?
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Math for everyoneN°197Dec 04, 2020Writing a Wikipedia article—why not you?
Whenever people have a question or want to check a concept, Wikipedia has become the first port of call for many. But who writes these articles? Can they be trusted? Above all, how can you write one yourself?
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Math for everyoneN°76Nov 06, 2020The on-line encyclopedia of integer sequences
Founded in 1995 by British-American Neil Sloane, the OEIS (On-Line Encyclopedia of Integer Sequences) is a collaborative website where anyone can add a new way of generating a sequence of numbers, together with its known properties and the questions it raises. Nearly 400,000 sequences can be browsed there!
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Math for everyoneN°74Apr 17, 2020Graceful roulettes
Drawing paper, tracing paper, colored pencils, a little curiosity and some practical ingenuity... You now have everything you need to explore roulettes and rediscover a now-forgotten classic of dynamic geometry: plane-on-plane motion.
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Math for everyoneN°74Apr 16, 2020Parabolas and catenaries
The parabola and the catenary are two grand dames of geometry, both with the same open U-shape. The first was studied by the Greeks 2,500 years ago; the second, "only" 330 years ago, by 17th-century mathematicians.
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Math for everyoneN°74Apr 16, 2020The endless variety of glissettes
Roulettes do not exhaust the possibilities offered by plane-on-plane motion—or the geometric wonders hidden within it. Armed once again with drawing paper, tracing paper and pens, we continue our exploration with some somewhat overlooked curves: glissettes.
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Math for everyoneN°184Sep 21, 2018The Bohemian dome
You are certainly familiar with the torus, obtained by rotating a circle about an axis lying in the circle's plane. But what happens if we change the circle's position? We obtain a surface that has long been known and used in architecture...
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