10. It’s Fermat’s birthday
9. I didn’t know whether “i” is the square root of -1 or “i” are the square root of -1.
8. I accidentally divided by 0 and my paper burst into flames.
7. I had too much pi and got sick.
6. I could only get arbitrarily close to my textbook.
It’s stuck inside a Klein bottle.
4. Someone already published it, so I didn’t bother to write it.
3. My 4-dimensional dog ate it.
2. I have a solar calculator and it was cloudy yesterday.
1. There wasn’t enough room to write it in the margin.