What is the meaning of $:s$ after a mathematical symbol?

Question

I started to read a book about regression analysis, and I faced this latex notation:

$e_i$:s, where $e_i$ is a residual.

What is the meaning of "$:s$"?

Answer 1

This is how the plural form of "$e_i$" is denoted, usually (I think) in the writing of European mathematicians. Consider the alternatives:

• $e_i$s — ugly and very likely to confuse
• $e_i$'s — looks like grocer's apostrophe

• $e_i$:s — looks odd if you're unfamiliar with it

• "numbers/vectors $e_i$" is what I would likely use