Great overview of the earlier history of unit standardization.

Gives particular emphasis to the invention of gauge blocks.

Ciro Santilli's preferred visualization of the real projective plane is a small variant of the standard "lines through origin in ${\mathbb{R}}^3$".

Take a open half sphere e.g. a sphere but only the points with $z>0$.

Each point in the half sphere identifies a unique line through the origin.

Then, the only lines missing are the lines in the x-y plane itself.

For those sphere points in the circle on the x-y plane, you should think of them as magic poins that are identified with the corresponding antipodal point, also on the x-y, but on the other side of the origin. So basically you you can teleport from one of those to the other side, and you are still in the same point.

Ciro likes this model because then all the magic is confined just to the $z=0$ part of the model, and everything else looks exactly like the sphere.

It is useful to contrast this with the sphere itself. In the sphere, all points in the circle $z=0$ are the same point. But this is not the case for the projective plane. You cannot instantly go to any other point on the $z=0$ by just moving a little bit, you have to walk around that circle.

Definition: "easy" number theory learnt in primary school, notably the operations of addition, subtraction, multiplication and division.

Evil company that desecrated the beauty created by Sun Microsystems, and was trying to bury Java once and or all in the 2010's.

Their database is already matched by open source e.g. PostgreSQL, and ERP and CRM specific systems are boring.

Oracle basically grew out of selling one of the first SQL implementations in the late 70's, and notably to the United States Government and particularly the CIA. They did deliver a lot of value in those early pre-internet days, but now open source is and will supplant them entirely.

Solving differential equations was apparently Lie's original motivation for developing Lie groups. It is therefore likely one of the most understandable ways to approach it.

It appears that Lie's goal was to understand when can a differential equation have an explicitly written solution, much like Galois theory had done for algebraic equations. Both approaches use symmetry as the key tool.