## Sekai 🌐 🗺

Sekai is the kanji for 世界, meaning “the world”. That’s a great word because of the scale it designates.

## Polar Rose in Julia

Background I’m doing exercise 4.9 of Think Julia, which asks for a function for a polar rose using Luxor’s turtle graphics. Difficulties Work out the geometric structure of the family of polar roses. The key is to construct some auxiliary isoceles triangles and work out the angles between them. One sees that they are parametrized by two varaibles n and k. n: number of petals k: petal increment constraint: k ≠ n ÷ 2 Handle the case when gcd(n, k) > 1, i. [Read More]

## Import Once in Julia

Background I’m reading Think Julia to repick the language again. To import a package/module, we using Foo. To run a script within the REPL, we include("foo.jl"). This is useful for user-defined functions, which are usually multi-line. Problem How to avoid importing a module/including a file twice? Solution Here’s a modified code from the book to give an example. polygon.jl depends on the module :ThinkJulia, and it defines a function which moves the object t along a polygon-shaped closed loop. [Read More]

## Deploy Hugo Theme exampleSite With Github Actions

Motivation To test PRs on the upstream of a Hugo theme by setting up a testing branch. Goal To deploy a forked GitHub repo for a Hugo theme with exampleSite to GitHub Pages using GitHub Actions. The whole article is based on my fork of Hugo Future Imperfect Slim. References GitHub Actions for Hugo A Stack Overflow question showing pwd in GitHub Actions A Hugo Discourse post about testing exampleSite Difficulties I had failed for about ten times before I got the job done. [Read More]

## First Experience with ASP.NET Core MVC & PostgreSQL

Goal To build an ASP.NET Core 5 MVC web app linked with a PostgreSQL. Motivation SQL Server is proprietary. SQLite used in Microsoft’s ASP.NET Core 5 MVC tutorial isn’t made for web apps. MySQL doesn’t perform well with concurrent read-writes. It’s dual-licensed like GitLab. Some users find PostgreSQL cost-effective. Useful tutorials MS’s tutorial in item 2 above. Wes Doyle’s YouTube video goes through the steps MS’s tutorial for Razor Pages with EF Core migrations Steps Create a superuser in the database. [Read More]

## Deleted Question on Semi-Simple and Projective but not Injective Module

A backup of a deleted PSQ : https://math.stackexchange.com/q/3955443/290189 OP : irfanmat It has a detailed answer by Atticus Stonestrom. It’s pity that his post got deleted. As there’s no reason for undeletion, I’m posting it here so as to preserve the contents. Question body Is there a semi-simple and projective but not injective module? I will be glad if you help. Response(s) In the non-commutative case, the answer is yes. [Read More]

## Another Math.SE Double Integral Using Polar Coordinate

I wanted to post the following answer to a question on double integral on Math.SE, but someone had submitted his work before I finished typing. As a result, I’m posting this on my personal blog. Let $r = \sqrt{x^2+4y^2}$ and $t = \begin{cases} \tan^{-1}(2y/x) &\text{ if } x > 0 \\ \pi/2 &\text{ if } x = 0. \end{cases}$ Then $\begin{cases} x &= r \cos t \\ y &= (r \sin t)/2 \end{cases}$ and $D = { (r,t) \mid r \ge 0, t \in [\pi/4, \pi/2] }$. [Read More]

## Ways to Draw Diagrams Displayed on Math.SE

I wanted to start a meta question, but I don’t see a point of that after viewing some related posts listed at the end of the next subsection. Intended question You may vote on your preferred way. Ways Advantages Disadvantages AMScd supported on Math.SE for a long time no diagonal arrowssyntax less well-knownTwo-way arrows $\rightleftarrows$ look odd array supported on Math.SE for a long timeeasier syntaxwrite basic diagonal arrows like $\nearrow$ fine tuning spacing is harddiagonal arrows only work for neighboring nodes ASCIIFlow WYSIWYG interface lines are rendered as slashes in code TikZ well known syntaxcan draw pretty diagrams not supported on SE, need to import as picture others? [Read More]

## Combat Procrastination

My notes for a YouTube video about procrastination on one hearing only. Why? Boring tasks. It’s necessary? Yes: Make it fun. Get a study partner. Find a more energetic time. No: Get rid of it. Surrounding environment. Identify someone who procrastinates in your circle of influence. Distance yourself from them, or Be aware of what their behavior. Perfectionism [Read More]

## GIMP w'ont Start after Ubuntu Distro Upgrade

Problem After upgrading to Ubuntu 20.04 form 18.04, I wasn’t able to start GIMP 2.10. The message was in the same format as that in a related Ask Ubuntu question, except that the version numbers were greater. I attempted installing the package gegl, which was irrelavant to the problem. Analysis I found the answers mentioning the package libgegl helpful. \$ dpkg -l | grep gegl ii gegl 0.4.22-3 amd64 Generic Graphics Library Test Program ii libgegl-0. [Read More]