Learning math sure isn't useless, and it seems to mostly consist of thinking about useless or nonexistent things.

I learned a lot of math (undergraduate major), and while it entertained me, it has been almost completely useless in my life. And the forms of math I believe to be most useful and wish I'd learned instead (statistics) are useful because they are so directly applicable to the real world.

What useful math have you learned that doesn't involve reference to useful or existent things?