This is the repository for the Lean sessions of the September 2023 Hausdorff School in Bonn.

Do the following:

• Install Lean 4 and VS Code following these instructions.

• Open a new terminal (we recommend git bash on Windows, which was installed as part of git in the first step).

• Go the the directory where you would like this package to live.

• Run git clone https://github.com/fpvandoorn/HausdorffSchoolLean.git.

• Run cd HausdorffSchoolLean

• Run lake exe cache get

  • On Windows, if you get an error that starts with curl: (35) schannel: next InitializeSecurityContext failed it is probably your antivirus program that doesn't like that we're downloading many files. The easiest solution is to temporarily disable your antivirus program.

• Launch VS Code, either through your application menu or by typing code .. (MacOS users need to take a one-off extra step to be able to launch VS Code from the command line.)

• If you launched VS Code from a menu, on the main screen, or in the File menu, click "Open folder" (just "Open" on a Mac), and choose the folder HausdorffSchoolLean (not one of its subfolders).

• Test that everything is working by opening HausdorffSchool/Test.lean. It is normal if it takes 10-40 seconds for Lean to start up.

• More files will be added to this repository during the tutorial. The first exercises are in HausdorffSchool/Session1_Basics/01Calculating.lean.

If you have already followed the steps above, and want to get the latest exercises, open a terminal in your local copy of this repository (e.g. cd HausdorffSchoolLean) and then run git pull. This gives you the new exercises.

If you have trouble installing Lean, you can use Lean directly in your browser using Github codespaces. This requires a Github account. If you are signed in to Github, click here:

• Make sure the Machine type is 4-core, and then press Create codespace
• After 1-2 minutes you see a VSCode window in your browser. However, it is still busily downloading mathlib in the background, so give it another few minutes (5 to be safe) and then open a .lean file to start.

Gitpod is an alternative to codespaces that is slightly inconvenient, since it requires you to verify your phone number.

Click this button to get started:

This creates a virtual machine in the cloud, and installs Lean and Mathlib. It then presents you with a VS Code window, running in a virtual copy of the repository. You can update the repository by opening a terminal in the browser and typing git pull followed by lake exe cache get as above.

Gitpod gives you 50 free hours every month. When you are done working, choose Stop workspace from the menu on the left. The workspace should also stop automatically 30 minutes after the last interaction or 3 minutes after closing the tab.

To restart a previous workspace, go to https://gitpod.io/workspaces/.

You can find the textbook that we will use online in html format or as a pdf document.