I wrote my Ph.D. dissertation in applied math with a Pentel 0.5 mm. Soooo, there's also a Pentel 0.7?? GOOD. Thanks!

Pentel makes 0.3, 0.4, 0.5, 0.7, 0.9mm

After trying all of them, I really gravitate to 0.4 and 0.7 depending on what I'm doing with it.

0.7 is great for general writing and notation, while 0.4 is better for drawing figures.