You're (probably) correct, considering that the specimen used Ø instead of ∅ (see https://braileinstitute.app.box.com/s/gw2s8k5kfc6t1uabkiyqlc...), and the font do not have the null symbol.

(also like the German commenter the null set is an empty set = {}, I only encountered ⌀ as diameter symbol, actually)

Wow, what ever that website is doing manages to completely and utterly break my gpu rendering in a way I haven't seen a site do in over a decade. It just paints the text over whatever was there last, including other windows above my browser.

No way, DK and NO aren't that big, and you learn about empty sets in HS in most of the world. I'd wager there are currently more Indian high schoolers than people in DK/NO.

In Germany (>20 years ago) we learned to use {}.

> you learn about empty sets in HS in most of the world

lol no.