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I'm fairly certain they were discovered, as those concepts exist regardless of whether minds are there to come up with them.

Yes, which is also why no one creates music, they merely discover it. All possible combinations of soundwaves already exists as a potential, and musiciancs merely choose which ones to play.

See, I think they were invented. I think numbers are part of an abstract universe of mathematics which we invented, which only happens to correspond to reality partly because we intended it to and partly because we just got lucky.

To be more serious than with Varg; that does raise the question of whether all of it was invented at once* - as the rules were created and thus all potential "discoveries" were already solidified - or when they were "discovered". Discovered here being used to refer to when humans first gained knowledge of them.
I'm not really sure if I'm communicating my question properly, but whatevs. Also, I think this issue might be more caused by language than the concepts it's trying to refer to. In the physical world, the distinction between discovery and creation is rather distinct, but I'm not sure if this is the case in thoughtspace (actual english word for this, plx?)

Fun fact: Invent is derived from a latin word for discovery.

*Ignoring different branches of mathematics