The application During this year’s MOP, we used the following procedure to divide some of our students into two classes: Let $latex {p = 7075374838595186541578161}&fg=000000$ be prime. Take the letters in your name as it appears on the roster, convert them with A1Z26 and take the sum of cubes to get a number $latex {s}&fg=000000$.… Continue reading A stupid “real-life” application of quadratic reciprocity
Category: Elementary Math
A proof of Poncelet Porism with two circles
Brian Lawrence showed me the following conceptual proof of Poncelet porism in the case of two circles, which I thought was neat and wanted to sketch here. (This is only a sketch, since I'm not really defining the integration.) Let $latex {P}&fg=000000$ be a point on the outer circle, and let $latex {Q}&fg=000000$ be the… Continue reading A proof of Poncelet Porism with two circles
New handout: Intro to Proofs for the Morbidly Curious
Downloadable at https://web.evanchen.cc/handouts/NaturalProof/NaturalProof.pdf. I don't know why I thought to write this, but it's been bugging me for a year or two now that I've never seen the answer to "what is a proof" written out quite this way. So here you go. It's a bit weird for me to be writing an article that… Continue reading New handout: Intro to Proofs for the Morbidly Curious
Circular optimization
This post will mostly be focused on construction-type problems in which you're asked to construct something satisfying property $latex {P}&fg=000000$. Minor spoilers for USAMO 2011/4, IMO 2014/5. 1. What is a leap of faith? Usually, a good thing to do whenever you can is to make ``safe moves'' which are implied by the property $latex… Continue reading Circular optimization
A story of block-ascending permutations
I recently had a combinatorics paper appear in the EJC. In this post I want to brag a bit by telling the ``story'' of this paper: what motivated it, how I found the conjecture that I originally did, and the process that eventually led me to the proof, and so on. This work was part… Continue reading A story of block-ascending permutations
Joyal’s Proof of Cayley’s Tree Formula
I wanted to quickly write this proof up, complete with pictures, so that I won't forget it again. In this post I'll give a combinatorial proof (due to Joyal) of the following: Theorem 1 (Cayley's Formula) The number of trees on $latex {n}&fg=000000$ labelled vertices is $latex {n^{n-2}}&fg=000000$. Proof: We are going to construct a… Continue reading Joyal’s Proof of Cayley’s Tree Formula
Positive Definite Quadratic Forms
I'm reading through Primes of the Form $latex {x^2+ny^2}&fg=000000$, by David Cox (link; it's good!). Here are the high-level notes I took on the first chapter, which is about the theory of quadratic forms. (Meta point re blog: I'm probably going to start posting more and more of these more high-level notes/sketches on this blog… Continue reading Positive Definite Quadratic Forms
Formal vs Functional Series (OR: Generating Function Voodoo Magic)
Epistemic status: highly dubious. I found almost no literature doing anything quite like what follows, which unsettles me because it makes it likely that I'm overcomplicating things significantly. 1. Synopsis Recently I was working on an elegant problem which was the original problem 6 for the 2015 International Math Olympiad, which reads as follows: Problem… Continue reading Formal vs Functional Series (OR: Generating Function Voodoo Magic)
New algebra handouts on my website
For olympiad students: I have now published some new algebra handouts. They are: Introduction to Functional Equations, which cover the basic techniques and theory for FE's typically appearing on olympiads like USA(J)MO. Monsters, an advanced handout which covers functional equations that have pathological solutions. It covers in detail the solutions to Cauchy functional equation. Summation,… Continue reading New algebra handouts on my website
Miller-Rabin (for MIT 18.434)
This is a transcript of a talk I gave as part of MIT's 18.434 class, the ``Seminar in Theoretical Computer Science'' as part of MIT's communication requirement. (Insert snarky comment about MIT's CI-* requirements here.) It probably would have made a nice math circle talk for high schoolers but I felt somewhat awkward having to… Continue reading Miller-Rabin (for MIT 18.434)
Power Overwhelming 