Coursera-dl is a wonderful tool but authentication doesn’t work out of the box. It will work after adding cauth option in coursera-dl.conf as here.

WIFI connected but no internet for Ubuntu

As title, the connection is okay after reboot but suddenly no internet connection but still has a wifi connection. This happens often after some wifi signal hiccup. It turns out that it seems to be ipv6 problems. After setting ipv6 to “ignore” and disable and enable connection, the internet is back nicely.

php came back to bite me

I came across trouble with my WordPress site because dropbox hanged. I reinstalled everything and forgot the PHP problem I had earlier. I ended up wasting a day trying to figure out what happened again. I still can’t figure out why php7.4 doesn’t work with the latest WordPress upgrade. Maybe something was wrong with the…

Can’t connect to AWS from ec2 interface

Came across this for one of my WordPress sites. I tried to stop and restart and service. And still can’t reconnect to my server. I ended up creating a new server and tried to recover from an earlier copy. The WordPress site ended up completely broken and it took me five hours to reinstall everything….

surfshark vpn

surfshark-vpn doesn’t seem to work for Linux somehow for several months. At least I thought it was just because I was overseas. Surfshark itself seems to be working. Just the script itself doesn’t seem to be able to connect to any server anymore. Manual configuration with OpenVPN works. But DNS Leak test from SurfShark failed…

16×9 Beamer setting

To use 16×9 aspect ratio for Beamer, we can simply change \documentclass{beamer} to \documentclass[aspectratio=169]{beamer} However, Bakoma won’t display correctly afterward. Go to Option -> Document Properties -> Paper. Just change paper width and paper height to 454bp and 255bp should make things work.

Skew-symmetric matrix representation of cross product

We can define Note that we have and Triple product expansion We will show the above with the triple product expansion: Proof: (1)   Similarly for the and components. Proof of Note that for any     thus Proof of For any (2)   Thus,

Rodrigues’ rotation formula

The rotation matrix for rotating an object along normal direction with angle is given by where such that We can easily validate that the equation is correct, note that as desired. And for any vector perpendicular to as desired as well. Compute and from  Note that  Thus, . Moreover, since , we can compute as…