The 3D regular convex polyhedrons are super famous, have the name: Platonic solid, and have been known since antiquity. In particular, there are only 5 of them.

The counts per dimension are:

The cool thing is that the 3 that exist in 5+ dimensions are all of one of the three families:Then, the 2 3D missing ones have 4D analogues and the sixth one in 4D does not have a 3D analogue: the 24-cell. Yes, this is the kind of irregular stuff Ciro Santilli lives for.

A convex regular polyhedron.

Their beauty is a classification type result as stated at classification of regular polytopes.

No good per-sub way as of 2022:

Just a circle.

Take ${\mathbb{R}}^2$ with a line at $x=0$. Identify all the points that an observer

For some reason, Ciro Santilli is mildly obsessed with understanding and visualizing the real projective plane.

To see why this is called a plane, move he center of the sphere to $z=1$, and project each line passing on the center of the sphere on the x-y plane. This works for all points of the sphere, except those at the equator $z=1$. Those are the points at infinity. Note that there is one such point at infinity for each direction in the x-y plane.

Important 4D spaces:

${\mathbb{R}}^n$ with extra structure added to make it into a metric space.

Quora is crap in many, many senses, but in part due to some bad Stack Overflow policies, it is the best crap we've got for certain (mostly useless) subjects. Until OurBigBook.com dominates the world.

The worst thing about quora is that you cannot subscribe only to certain subjects on your feed. Quora just keeps pumping shit you never subscribed to, no matter what. Ciro, for sport, unfollowed every single idiotic subject it was proposing, but it didn't work, sooner or later Quora just keeps pumping more shit back. Mind you, some of that shit is fun. But it's still shit. Though on second thought, YouTube also randomly decides to reset Ciro's humongous "don't recomend this shitty channel" choices from time to time, which is not much different...

Other terrible things, they just seem to have an incredible ability of making the website worse and more annoying over time! Truly amazing:

Quora is getting so bad that it is basically being killed by Reddit, especially after they lifted the 6-month hard thread lock...

See also: cirosantilli.com/china-dictatorship/quora for a coverage of the intense pro-CCP astroturfing present on the website.

The best place to get answers to programming questions as of 2019. Google into Stack Overflow is always the best bet.

An overview of Ciro Santilli's Stack Overflow contribution can be found at: Ciro Santilli's Stack Overflow contributions.