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Recent Posts
 Next Week in Jerusalem: Special Day on Quantum PCP, Quantum Codes, Simplicial Complexes and Locally Testable Codes
 Happy Birthday Ervin, János, Péter, and Zoli!
 My Mathematical Dialogue with Jürgen Eckhoff
 Test Your Intuition (23): How Many Women?
 Happy Birthday Richard Stanley!
 Influence, Threshold, and Noise
 Erdős Lectures 2014 – Dan Spielman
 Answer to Test Your Intuition (22)
 Test your intuition (22): Selling Two Items in a Bundle.
Top Posts & Pages
 Polymath 8  a Success!
 The KadisonSinger Conjecture has beed Proved by Adam Marcus, Dan Spielman, and Nikhil Srivastava
 Believing that the Earth is Round When it Matters
 A Problem on Planar Percolation
 Next Week in Jerusalem: Special Day on Quantum PCP, Quantum Codes, Simplicial Complexes and Locally Testable Codes
 Test Your Intuition (23): How Many Women?
 My Mathematical Dialogue with Jürgen Eckhoff
 New Ramanujan Graphs!
 NavierStokes Fluid Computers
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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 twodistance 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
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
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! FrenchIsreali Meeting … Continue reading
Test Your Intuition (21): Auctions
You run a singleitem 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 Auctions, Roger Myerson, Test your intuition
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 Erosion, J. F. C. Kingman, Probability, S. E. Volkov
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 Earth, Geology, Lord Kelvin, Test your intuition
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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.
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 Ramanujan graphs
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 Oz, Probability, Test your intuition
14 Comments