Primitive recursive function by Ciro Santilli 35 Updated +Created
In intuitive terms it consists of all integer functions, possibly with multiple input arguments, that can be written only with a sequence of:
for (i = 0; i < n; i++)
and such that n does not change inside the loop body, i.e. no while loops with arbitrary conditions.
n does not have to be a constant, it may come from previous calculations. But it must not change inside the loop body.
Primitive recursive functions basically include every integer function that comes up in practice. Primitive recursive functions can have huge complexity, and it strictly contains EXPTIME. As such, they mostly only come up in foundation of mathematics contexts.
The cool thing about primitive recursive functions is that the number of iterations is always bound, so we are certain that they terminate and are therefore computable.
This also means that there are necessarily functions which are not primitive recursive, as we know that there must exist uncomputable functions, e.g. the busy beaver function.
Adding unbounded while loops of course enables us to simulate arbitrary Turing machines, and therefore increases the complexity class.
More finely, there are non-primitive total recursive functions, e.g. most famously the Ackermann function.
Brooks's law by Ciro Santilli 35 Updated +Created
Video 1.
The Misty Mountains Cold Scene from The Hobbit: An Unexpected Journey (2012)
Source.
I will take each and every one of these dwarves over an army from the Iron Hills. For when I called upon them they answered. Loayalty. Honour. And willing heart. I can ask no more than that.
Keep debug notes by Ciro Santilli 35 Updated +Created
When debugging complex software, make sure to keep notes of every interesting find you make in a note file, as you extract it from the integrated development environment or debugger.
Especially if your memory sucks like Ciro's.
This is incredibly helpful in fully understanding and then solving complex bugs.
High pressure by Ciro Santilli 35 Updated +Created
Video 1.
Something weird happens when you keep squeezing by Vox (2023)
Source. Sodium becomes liquid when you compress it. Weird.
Radio by Ciro Santilli 35 Updated +Created
Fermi gas by Ciro Santilli 35 Updated +Created
Bose-Einstein condensate by Ciro Santilli 35 Updated +Created
Inward Bound by Abraham Pais (1988) page 282 shows how this can be generalized from the Maxwell-Boltzmann distribution
Type of material by Ciro Santilli 35 Updated +Created
Glass by Ciro Santilli 35 Updated +Created
Symplectic group by Ciro Santilli 35 Updated +Created
Intuition, please? Example? mathoverflow.net/questions/278641/intuition-for-symplectic-groups The key motivation seems to be related to Hamiltonian mechanics. The two arguments of the bilinear form correspond to each set of variables in Hamiltonian mechanics: the generalized positions and generalized momentums, which appear in the same number each.
Seems to be set of matrices that preserve a skew-symmetric bilinear form, which is comparable to the orthogonal group, which preserves a symmetric bilinear form. More precisely, the orthogonal group has:
and its generalization the indefinite orthogonal group has:
where S is symmetric. So for the symplectic group we have matrices Y such as:
where A is antisymmetric. This is explained at: www.ucl.ac.uk/~ucahad0/7302_handout_13.pdf They also explain there that unlike as in the analogous orthogonal group, that definition ends up excluding determinant -1 automatically.
Therefore, just like the special orthogonal group, the symplectic group is also a subgroup of the special linear group.
Ciro's Edict #4 / Advances by Ciro Santilli 35 Updated +Created

Pinned article: ourbigbook/introduction-to-the-ourbigbook-project

Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.
Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!
We have two killer features:
  1. topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculus
    Articles of different users are sorted by upvote within each article page. This feature is a bit like:
    • a Wikipedia where each user can have their own version of each article
    • a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
    This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
    Figure 1.
    Screenshot of the "Derivative" topic page
    . View it live at: ourbigbook.com/go/topic/derivative
  2. local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:
    This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
    Figure 5. . You can also edit articles on the Web editor without installing anything locally.
    Video 3.
    Edit locally and publish demo
    . Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension.
  3. https://raw.githubusercontent.com/ourbigbook/ourbigbook-media/master/feature/x/hilbert-space-arrow.png
  4. Infinitely deep tables of contents:
    Figure 6.
    Dynamic article tree with infinitely deep table of contents
    .
    Descendant pages can also show up as toplevel e.g.: ourbigbook.com/cirosantilli/chordate-subclade
All our software is open source and hosted at: github.com/ourbigbook/ourbigbook
Further documentation can be found at: docs.ourbigbook.com
Feel free to reach our to us for any help or suggestions: docs.ourbigbook.com/#contact