# Math mode in LaTeX

This is my first time using LaTeX and I am running into the following issue:

I am trying to denote a set of functions x_t which belong to K but for some reason the entire sentence gets italicized. I have attached screenshots of my code as well as the compiled result. Can anyone help me fix this? Cheers

• Welcome! Please paste code here rather than a picture of it - preferably in the form of a small but complete document which we can use to reproduce the issue. – cfr Oct 14 '16 at 1:19
• Welcome to the site! Don't worry, you're not the first and you won't be the last :) It's important to understand that some commands can only appear in math mode, such as _ and \mapsto. This is why LaTeX is inserting the missing $ signs, but it's put everything in math mode, hence your problems. The next and most important thing to understand is that $ ... $ shouldn't be used as a way of "mathifying" individual glyphs, or getting special characters, it sets up math mode, which is where the entire equation/relation/expression/object should go, in order to get the right spacing &c. – Au101 Oct 14 '16 at 1:33 ## 2 Answers That code is surely giving you errors when you compile, which will be telling you at least something about the problem. TeX will complain about missing $ signs and add them in, but it can only guess where to put them.

You want everything from f_{t} to \mathbb{R}between one set of $...$ and then from x_{t} to the end between another set. f_{t} : etc. is maths and you have it set as text. Similarly, \mapsto etc.

$f_{t} \colon \mathcal{K} \mapsto \mathbb{R}$
...
$x_{t} \in \mathcal{K}$


EDIT Use \colon as suggested by Mico.

• That's OK. But if you paste code we can copy-paste, it is a lot easier to help than giving us a screen shot. – cfr Oct 14 '16 at 1:30
• +1. A minor point: Use \colon instead of :. – Mico Oct 14 '16 at 7:53

<rant>
As a regular user of Mathematics.SE, I'm really impressed about the poor quality of TeX/LaTeX input. I acknowledge that some compromise with the limited possibilities offered by MathJax is necessary, but when I see

$\rm\ (p-1)!\ mod\ p\:$


written by a high rep user, my heart bleeds. Apart from the personal choice of using upright letters everywhere, typing \mod{p} shouldn't be too difficult.

One of the most common errors, that I also find in students' typescripts, is believing that $...$ is just a way to enable special characters. It is not.

Everything which is math must be typed in as math, even if it is a standalone variable. In MathJax and, a fortiori, in LaTeX.

</rant>

So

We model our decision set, $\mathcal{K}$, as a convex set
in Euclidean space. Furthermore, we define our convex loss
function as $f_{t}\colon \mathcal{K} \rightarrow \mathbb{R}$.
At any finite time $t$, we denote our player's decision
as $x_{t} \in \mathcal{K}$.


Style manuals say the colon should not be used after "as". Also \mapsto is the wrong symbol for functions, it is used to denote a function's action on elements. Also no space should ever precede a comma.

• so, how do you propose getting mathematicians/latex users to read and observe basic introductory material? (i agree with your rant and have some good examples, but i won't name names since that's politically unwise.) – barbara beeton Oct 14 '16 at 13:14
• @barbarabeeton Rejecting badly written manuscripts could be a start. ;-) – egreg Oct 14 '16 at 14:39
• Er ... who is this rant aimed at? Neither the OP nor I wrote that. – cfr Oct 14 '16 at 15:32
• @cfr Did I say that? I meant to say I've seen much worse code than the OP's. – egreg Oct 14 '16 at 15:40