Thank you for your suggestions on the swag ideas. For our math community, we'll be sending top users (first 2 pages of all time, sorted by rep) coffee mugs and a writing pad.
Below is a mockup up of the design.
For the design, I think it's more appropriate to emphasize the math aspect; so I'm using Euler's Identity as the main focus.
Of course, I'm not a mathematician, if you have a suggestion for another equation/theorem, please let me know. Please do keep in mind, the space is somewhat limited, so I prefer nothing too complex(from a design POV).