bad formating of ff in math mode

Question

I'm trying to write something like: $affinity(x)$, but in the PDF I see some small spacing between the two f's.

I tried $af{}firm(x)$ or $aff{}irm(x)$, but nothing works.

Answer 1

$\mathit{affinity}(x)$

Answer 2

$affinity$ stands for the product of the variables a, f, f, i, …. Wordlike functions are usually written in an upright font, usually with \DeclareMathOperator:

\usepackage{amsopn}
\DeclareMathOperator{\affinity}{affinity}

Answer 3

How do you usually write maps, functions and operators in your math text? Italic as well?

Math operators are commonly written in upright letters, in contrary to variables which have italic shape. I would use \mathrm instead of \mathit or better

\DeclareMathOperator{\affinity}{affinity}

of amsmath, and write \affinity in the formulas. Here's a complete example, just compare:

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\affinity}{affinity}
\begin{document}
\[
  \affinity(x) \neq \mathit{affinity}(x) \neq y(x),
\]
\[
  \max_f \lim_a af = a\cdot f
\]
\end{document}

Output: