Nilpotent Matrices

October 5th, 2009 by Jeremy No comments »

Show that if T^i=0, where T operates on an n-dimensional vector space V, then T^n=0.

Binary Expansions

September 21st, 2009 by Jeremy No comments »

Are the binary expansions of all rational numbers eventually periodic? Answer with proof.

Never Invertible?

September 17th, 2009 by Jeremy No comments »

The product of two rectangular matrices sized mxn and nxm respectively with m

Bug on a Triangle!

September 15th, 2009 by Jeremy No comments »

A bug moves along segments on a triangle. From any particular vertex, he has a \frac{1}{2} chance of moving to either of the other vertices. If traversing from one vertex to another constitutes a step, what is the probability that the bug moves to its original location after 12 steps?

Hint: there is a slightly easier (albeit less rigorous) heuristic approach over the traditional one.

Taken from an AIME exam

Interesting clock question

September 8th, 2009 by Jeremy No comments »

Just when you thought you saw all of them!

A man leaves his house between 4 pm and 5 pm. He returns between 5 pm and 6 pm only to discover that the hour hand and the minute hand have exactly interchanged places! Deduce the time the man left his house.

*Source: India National Olympiad 1986

Modular Arithmetic #1

July 21st, 2009 by Jeremy No comments »

Ex 3.2.1
Prove that any subset of 55 numbers chosen from the set of integers from one to a hundred must contain two numbers differing by 9.
Solution
There are nine congruence classes modulo 9. By the pidgeonhole principle seven of the fifty-five chosen are in the same congruence class. Label them  a_{1} through a_{9} such that each term is greater than the previous term. Since a_{i+1} \equiv a_i \mod n,  a_{i+1} - a_i is a multiple of nine. There must be two elements that differ by nine because, otherwise, we must have two which differ by at least 6*18=108 which is greater than a hundred.

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Putnam Training Day

June 10th, 2009 by Jeremy No comments »

I will be training for the Putnam this upcoming year and will be posting an assortment of problems and my personal solutions here on this webpage. Each post will have the following format: Example, Problem, Solution.

Strategy:
Exploit Symmetry
Source:
Problem Solving Through Problems
Example:
Evaluate \int_0^{\frac{\pi}{2}}  \frac{dx}{1+(\tan{x})^{\sqrt{2}}}.
Example Solution:
Exploit the symmetry of the expression about the point (\frac{\pi}{4}, \frac{1}{2}). To see this symmetry is true, one only need verify that f(x)+f(\frac{\pi}{2}-x)=1 for all x, 0\leq x \leq\frac{\pi}{2}.
From symmetry, the expression evaluates to one half the area of the rectangle that encloses the function over the domain of the integral.
\int_0^{\frac{\pi}{2}}  \frac{dx}{1+(\tan{x})^{\sqrt{2}}}=\frac{\pi}{4}

Problem and my solution coming later today.

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