We often write "otherwise" in mathematics:

https://arxiv.org/html/1501.05438v2#:~:text=otherwise,-%2C

https://arxiv.org/html/1302.5008v3#:~:text=otherwise

https://arxiv.org/html/1501.05438v2#:~:text=otherwise,-%2E

In fact, I think that's the most common alternative, not "else".

"or else" works too but that's still three extra characters