# Blog entries with tag ‘Jots’

• Jots
• 随笔

## “趁热打铁”出自哪里？

“趁热打铁”是从法语翻译来的成语吗？

## A series about iterated cosine

Just some random proposition found on Twitter.

## Red envelopes

WeChat has this function to distribute red envelopes of random amount of money. The natural question to ask is how to sample the amount of money each person who opens the envelope should receive.

## Two (equivalent) definitions of computational indistinguishability, two (isomorphic) ways of doing hybrids

It wasn’t until recently did I realise there are two isomorphic ways (different styles) of writing hybrid proofs. They actually correspond to the two equivalent definitions of computational indistinguishability — one based on guessing and the other based on distinguishing. Actually, I’ve been writing both kinds of proofs subconciously.

## The two asymptotically equivalent definitions of IND-CPA

There are two asymptotically equivalent definitions of IND-CPA for secret-key encryption schemes. I think the one-challenge one makes the proof neater.

## Allen School 配发的 MacBook

UW CSE 的 PhD 要约信中提到新入学的研究生都会获得一个免费电脑（有多种选择），我选了（标准选项里的）MacBook。这篇博文记录我初探 macOS 的事情。作为探索结果，我居然又给微软报了一个 bug！

## Nyuwa mends the epigraph

Nyuwa, the mother goddess of Chinese mythology, mends the sky according to an ancient Chinese legend. Today, Nyuwa is asked to mend the epigraph of a continuous function over an open set so that the function becomes convex. What can we say about this?

## La composition soignée des formules mathématiques

It is widely spoken that the Equation Editor of Word (OneNote / Office) is inferior and harder to use than LaTeX. While this is true for a hardcore user, a lot of people spreading the word aren’t really understanding the thing and are just parroting others. I suspect that for a lot of them, the ability to efficiently typeset beautiful mathematical formulae is limited by knowledge and carefulness. People don’t seem to care enough to learn and correctly employ the tools at hand.

## In memory of a nightmare: solving linear equation over PID

I had a maths-related nightmare that (initially) was concerned about counting solutions of a linear equation modulo M with integral coefficients. Though the whole detail of the dream cannot be written down due to privacy reasons, at least I can write a simple introduction on solving this specific problem here. Update: An error in the original solution has been corrected. Another update the same day: The original solution is correct.

## Brain teaser: locks, keys and gifts

A brain teaser that remains open in Yao class WeChat group, up to the time of writing of this entry. See if it interests you and if you can solve it.

## NTFS DELETE_CHILD access and ‘the directory is not empty’

Someone wanted to know how to create a file that denies itself from being removed, yet found he could still remove the file even denying DELETE access from Everyone. The reason is that there is DELETE_CHILD access on its containing directory. How does this relate to ‘the directory is not empty’?

## How to watch movies on UNITED flights

a.k.a. How to be disappointed again by UA. I was visiting New York University and Carnegie Mellon University. Flying from Beijing to New York, from New York to Pittsburgh, from Pittsburgh to Beijing via Chicago, I chose United Airlines, one that suffers from previous scandals on ‘violently reaccomodating a customer’. It turns out that their in-flight entertainment system is also torturing! This entry was written during the flight from Chicago to Beijing.

## Rethinking base ‘conversion’

Base conversion is the conversion between notations of number, and NOT the numbers themselves. I discuss a common mistake on methods of base conversion, and a common mistake in thinking about ‘base conversion’ in computer programs.

## Chit-chat on Lagrange’s four-square theorem

Lagrange’s four-square theorem is a beautiful result in number theory. However, to the best of my knowledge, I haven’t encountered it often in theoretical computer science. Today’s entry discusses two interesting stories related to this theorem.

## Discount aux Galeries Lafayette and NP-completeness

I allowed myself some CHANEL products to ‘reward’ myself for the acceptance of my first cryptography paper. I also consider spending money at its best value an interesting game. A commonly-seen discount aux Galeries Lafayette Xidan (Beijing) leads to an obviously NP-complete problem. Haha!

## How to implement Frequented Visitees of Sina Weibo

An algorithmic imagination of how Sina Weibo could implement Frequented Visitees, a feature that gives users the serveral accounts which they visit the most often.

## A Piece of Art: Localisation of an Application Bar Button

How do you localise ‘Filter’ button for Mainland China?

## 谈“回”、“老家”和“回老家”

“回”“老家”四川对我来说是一种负担，我想回老家——回北京。

## Interviewed by THU IIIS

I was participating Advanced Assessment for Admission (AAA) of Tsinghua Union, passed the first phase and was interviewed by Insititute for Interdisciplinary Information Sciences of Tsinghua University. Here is a transcript of the interview.