KGS math club

A group of people on Kiseido Go Server Mathematics room.

The meaning of this page is to collect the problems posed there and save hints and solution suggestions. In order to write something, I'm afraid you need to register to the AoPS wiki first. After that you're good to go.

Adding problems should be quite straightforward with the copy-paste template in the wiki source. Please add <math>-tags (or dollar signs, it seems) where required, e.g. . Still, if you don't, somebody else will; all additions are appreciated.

KGS math problems

Added	Author	Problem	Solutions
2011-03-06	iceweasel	A 52-card deck is shuffled and cards are taken from the top and shown, one by one. You are forced to make a $1 bet that "the next card drawn will be black" once before the deck is emptied. Your only freedom is choosing when to make this bet, depending on what you've seen so far. What is the maximum expected gain from your bet? |style="background-color:rgb(220,230,255);" | |- valign="top" |2011-02-14 |style="background-color:rgb(220,230,255);" | warfreak2 | A regular tetrahedron formed from six thin sticks is completely infested with greenfly, which breed rapidly and spread along the sticks at 1mm per second. There are three ladybugs that can walk at up to 1.1 mm per second, eating the greenfly that they pass. How can they exterminate the greenfly? |style="background-color:rgb(220,230,255);" |[[KGS math club/solution_11_3| solution]] by Swifft |- valign="top" |Feb. 2011 |style="background-color:rgb(220,230,255);" | maproom | The number of ways to choose k things from n (n>2k) is equal to the number of ways to choose n-k things. Find a general way to pair up the k-member subsets with the (n-k)-member subsets such that each of the former is a subset of its partner. |style="background-color:rgb(220,230,255);" |[[KGS math club/solution_11_2| solution]] by iceweasel |- valign="top" |Jan. 2011 |style="background-color:rgb(220,230,255);" | maproom | How many dissimilar ways are there to arrange five points in the plane such that there are only two distinct distances between pairs of the points? |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_11_1| solution]] by Warfreak2 |- valign="top" |11.8.2010 |style="background-color:rgb(220,230,255);" | ghej |For the curve x^2 + x y + y^2 = 1, find the tangent that passes through the point (0,2). |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_10_1| solution ]] |- valign="top" |19.8.2009 |style="background-color:rgb(220,230,255);" | royu | You have the set {9, 99, 999, ...}. Show that given any natural number n not divisible by 2 or 5, n divides at least one element of the set. |style="background-color:rgb(220,230,255);" | [[KGS math club/hints_9_1| hint ]] [[KGS math club/solution_9_1| solution ]] |- valign="top" | 19.8.2009 |style="background-color:rgb(220,230,255);" | bourbaki | Suppose A and B are n x n matrices with real entries such that either A or B commutes with C = AB - BA. Prove that C is nilpotent, i.e. C^k = 0 for some integer k |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_8_1| solution ]] |- valign="top" | 18.7.2009 |style="background-color:rgb(220,230,255);" | taoyan | How many times do the clock hands (hour and minute) overlap between 11:59:59 before lunch and 00:00:01 at night? |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_7_1| solution ]] |- valign="top" | 27.7.2008 |style="background-color:rgb(220,230,255);" | royu StoneTiger | You have a collection of 11 balls with the property that if you remove any one of the balls, the other 10 can be split into two groups of 5 so that each weighs the same. If you assume that all of the balls have rational weight, there is a cute proof that they all must weigh the same. Can you find a proof? Can you find a way to extend the result to the general case where the balls have real weights? |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_6_1| solution ]] |- valign="top" |6.7.2008 |style="background-color:rgb(220,230,255);" | amkach |Prove or disprove: If P(x) is a polynomial (with non-zero degree) of one real variable and a and b satisfy$ (Error compiling LaTeX. Unknown error_msg)P^{(n)}(a) = P^{(n)}(b)P(a) = P(b), P'(a) = P'(b), P(a) = P(b)$, etc.), then a = b |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_5_1| solution ]] |- valign="top" |1.7.2008 |style="background-color:rgb(220,230,255);" | quimey |Assume$ (Error compiling LaTeX. Unknown error_msg)mn2a,b,c,dm=a^2+b^2, n=c^2+d^2)m*n24$squares. |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_4_1| solution ]] |- valign="top" |30.6.2008 |style="background-color:rgb(220,230,255);" | amkach |For$ (Error compiling LaTeX. Unknown error_msg)n \geq 2n-4n-2^n$$ (Error compiling LaTeX. Unknown error_msg)n-1(\pm1, \pm1, ..., \pm1)$. These hyperspheres are tangent to the hypercube and to each other. Then place an$ (Error compiling LaTeX. Unknown error_msg)n-2^n1n$is this central hypersphere contained within the hypercube? |style="background-color:rgb(220,230,255);" | [[KGS math club/solution_3_1| solution ]] |- valign="top" |21.6.2008 |style="background-color:rgb(220,230,255);" | amkach |Consider the two player game that begins with an even length sequence of positive integers. Each player, in turn, removes either the first or last of the remaining integers, ending when all the integers have been removed. A player's score is the sum of the integers that they removed; the winner is the player with the higher score (with a tie if equal scores). Show that Player One has a non-losing strategy, i.e., can always force a tie or a win. |style="background-color:rgb(220,230,255);" | [[KGS math club/hints_1_1|hints]] [[KGS math club/solution_1_1|solution]] [[KGS math club/solution_1_2|solution2]] |- valign="top" |20.2.2007 |style="background-color:rgb(220,230,255);" | StoneTiger |Does any member of the sequence$ (Error compiling LaTeX. Unknown error_msg)1, 4, 20, 80, ...2007$?	sigmundur
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