Author Archives: Gil Kalai

Andriy Bondarenko Showed that Borsuk’s Conjecture is False for Dimensions Greater Than 65!

The news in brief Andriy V. Bondarenko proved in his remarkable paper The Borsuk Conjecture for two-distance sets  that the Borsuk’s conjecture is false for all dimensions greater than 65. This is a substantial improvement of the earlier record (all dimensions … Continue reading

Posted in Combinatorics, Geometry, Open problems | Tagged , , , , | 2 Comments

Why is mathematics possible?

Spectacular advances in number theory Last weeks we heard about two spectacular results in number theory.  As announced in Nature, Yitang Zhang proved that there are infinitely many pairs of consecutive primes which are at most 70 million apart! This is a sensational achievement. … Continue reading

Posted in Computer Science and Optimization, Number theory, Open discussion, Philosophy, Updates, What is Mathematics | 15 Comments

Dan Mostow on Haaretz and Other Updates

Enlightenment at a red traffic light Wolf Prize laureate Prof. George Daniel Mostow made his greatest scientific breakthrough while driving. Haaretz tells the story of how Dan Mostow reached his breakthrough known as Mostow’s rigidity theorem. Congratulations, Dan! French-Isreali Meeting … Continue reading

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Test Your Intuition (21): Auctions

You run a single-item sealed bid auction where you sell an old camera. There are three bidders and the value of the camera for each of them is described by a certain (known) random variable: With probability 0.9 the value … Continue reading

Posted in Economics, Games, Test your intuition | Tagged , , | 4 Comments

Oz’ Balls Problem: The Solution

A commentator named Oz proposed the following question: You have a box with n red balls and n blue balls. You take out each time a ball at random but, if the ball was red, you put it back in the box and take out … Continue reading

Posted in Probability, Test your intuition | Tagged , , , | 1 Comment

Answer: Lord Kelvin, The Age of the Earth, and the Age of the Sun

            Yeshu Kolodni and Lord Kelvin The question In 1862, the physicist William Thomson (who later became Lord Kelvin) of Glasgow published calculations that fixed the age of Earth at between 20 million and 400 … Continue reading

Posted in Geology, Physics, Test your intuition | 4 Comments

Test your Intuition/Knowledge: What was Lord Kelvin’s Main Mistake?

The age of the earth (Thanks to Yeshu Kolodny) We now know that the age of the earth is 4.54±1% Billion years. From Wikipedea: In 1862, the physicist William Thomson (who later became Lord Kelvin) of Glasgow published calculations that … Continue reading

Posted in Controversies and debates, Geology, Physics, Test your intuition | Tagged , , , | Leave a comment

Indian Crested Porcupine

For a few days we had an Indian Crested Porcupine (דורבן) in our garden. It ate all the flowers and dug an impressive array of tunnels.

Posted in Updates | Tagged , , | 1 Comment

New Ramanujan Graphs!

Margulis’ paper Ramanujan graphs were constructed independently by Margulis and by Lubotzky, Philips and Sarnak (who also coined the name). The picture above shows Margulis’ paper where the graphs are defined and their girth is studied. (I will come back to the question … Continue reading

Posted in Algebra and Number Theory, Combinatorics, Open problems | Tagged | 10 Comments

Taking balls away: Oz’ Version

This post is based on a comment by Oz to our question about balls with two colors: “There is an interesting (and more difficult) variation I once heard but can’t recall where: You have a box with n red balls … Continue reading

Posted in Guest post, Probability, Test your intuition | Tagged , , | 14 Comments