[_Microft]

Very nice, thanks for sharing! Maybe show which upper or lower values are included in the intervals? A notation I am familiar with uses outward facing brackets if the value is not included in the interval. That always applies to infinity.

Applied to the cases here:

]-∞, -1] U [0.5, +∞[

The excluded interval in between becomes ]-1, 0.5[ then.

That’s how min (and analogously max) works, right? min(A, B) = [lo(A,B), lo (hi(A), hi(B))].

Edit: idea: copy a formula from the results section to the input field if the user clicks/taps on it.

[globular-toast]

I was also a bit confused by this. I thought the standard notation was round brackets, but maybe doesn't work well in ASCII?

[qbit42]

Round brackets are standard in the US but that notation is used in France and some other places.

[meindnoch]

  (0, 1)

Is this an twice-open interval or a 2D vector?

See, this is why Bourbaki introduced the ]0,1[ notation.

[streetfighter64]

Is there any reasonable situation where you'd be confusing a vector with an interval? Having done mathematical writing and grading of tests using both styles of notation (simply adapting to what was used at the institution), I can't say that I ever noticed any practical difference between them.

[adito]

From reading the linked paper[0], It explains closed interval only. "An interval union is a set of closed and disjoint intervals where the bounds of the extreme interval can be ±∞".

[0]: https://www.ime.usp.br/~montanhe/unions.pdf

[fouronnes3]

It's possible to support that but it makes the code very very much more complicated. I've decided early on to not support it. Would be a cool addition though!