[jefftk]

> What function do I use?

Traditionally you use sum, which gets you total utilitarianism. Some have advocated avg which gets you average utilitarianism. https://en.wikipedia.org/wiki/Average_and_total_utilitariani...

> root-mean-squared

Why?

> Can I perform arithmetic on utilities from two different agents?

This is called "interpersonal utility comparison", and there's a ton of literature on it. Traditionally utilitarians have accepted it, and without it ideas like "sum the utility across everyone" don't make sense.

[schoen]

I think the people you're responding to are taking the modus ponens for your modus tollens, saying that interpersonal utility comparisons are not obviously possible or meaningful, and so aggregating utilities across persons is also not obviously possible or meaningful.