[zombot]

Why does the `i` after `h_` need a curly but not after `\sum_`?

A pity that HN can't render TeX or MathJax or whatever.

[tempfile]

It doesn't. The rule for TeX is that single-character tokens don't need braces. I assume this is just an artefact of their particular muscle memory.

You can test it here, if you like: https://www.mathjax.org/#demo