I’ve read some math publications from around a century or so ago, and I wondered if I could pull off being a math professor if a time machine dropped me into a math department from the time. I think I’d come across as something of an autistic savant, ignorant of what contemporaries would think of as basic math but fluent in what they’d consider more advanced.
There are two things in particular that were common knowledge at the time that I would be conspicuously ignorant of: interpolation tricks and geometry.
People from previous eras knew interpolation at a deeper level than citing the Lagrange interpolation theorem, out of necessity. They learned time-saving tricks have since been forgotten.
The biggest gap in my knowledge would be geometry. Mathematicians a century ago had a far deeper knowledge of geometry, particularly synthetic geometry, i.e. geometry in the style of Euclid rather than in the style of Descartes.
Sometimes older math books use notation or terminology that has since changed. I imagine I’d make a few gaffs, not immediately understanding a basic term or using a term that wasn’t coined until later.
If I had to teach a class, I’d choose something like real and complex analysis. Whittaker & Watson’s book on the subject was first published in 1902 and remains a common reference today. The only thing I find jarring about that book is that “show” is spelled “shew.” Makes me think of Ed Sullivan. But I think I’d have a harder time teaching a less advanced class.