Mathjax syntaxes that don't look very good?

Question

These MathJax syntaxes look fine to me:

\require{cancel}\cancel{\Longleftrightarrow} $$\require{cancel}\cancel{\Longleftrightarrow}$$

\nLeftrightarrow $$\nLeftrightarrow$$

\nRightarrow $$\nRightarrow$$

\nLeftarrow $$\nLeftarrow$$

\require{cancel}\cancel{\Longrightarrow} $$\require{cancel}\cancel{\Longrightarrow}$$

\require{cancel}\cancel{\Longleftarrow} $$\require{cancel}\cancel{\Longleftarrow}$$

Now, what can we say about these Mathjax syntaxes?

\require{cancel}\cancel{\Leftrightarrow} $$\require{cancel}\cancel{\Leftrightarrow}$$

\not \Longleftrightarrow $$\not \Longleftrightarrow$$

\not \Longleftarrow $$\not \Longleftarrow$$

\not \Longrightarrow $$\not \Longrightarrow$$

Seems like it doesn't look very good to me ...(essentially the first two)

• Can these syntaxes be considered good?

• Can we do the long version of this syntaxis?

\nLeftrightarrow $$\nLeftrightarrow$$

Because,as it seems \nLongleftrightarrow syntax

$$\nLongleftrightarrow$$ doesn't work.

Answer 1

Illustration for the OP here, given our discussion above.

Here's an example of how \require{cancel} can be worth declaring at the start of the post. Please click on "edit" to see how I wrote this.

$\require{cancel}$

Provided $x\neq 1$, we have: $$\frac {x^2-1}{x^3-1} = \frac{\cancel{(x-1)}(x+1)}{\cancel{(x-1)}(x^2 + x + 1)} = \frac{x+1}{x^2 + x + 1}.$$

This is particularly useful in a longer post in which you plan to use \cancel multiple times.

Answer 2

I don't really understand the question. But I think at least part of the question is how to have a reasonable looking "long arrow" version of $\nLeftrightarrow$ and so on. The OP is aware of the cancel package, but the result of e.g. \cancel\iff$\cancel\iff$ IMO is suboptimal. Here I suggest a variant with some \hspace commands. I made the space taken out/put back in equal on both sides, so it should look approximately OK* in most browsers / math renderers. There is \require{cancel}$\require{cancel}$ at the start of this post.

1. $a\iff b$a\iff b

2. $a\cancel{\iff}b$a\cancel{\iff}b

3. $a\hspace{1em} \cancel{\hspace{-1em}{\iff}\hspace{-1em}}\hspace{1em} b$a\hspace{1em}\cancel{\hspace{-1em}{\iff}\hspace{-1em}}\hspace{1em}b

4. $a\implies b$a\implies b

5. $a\cancel{\implies}b$a\cancel{\implies}b

6. $a\hspace{1em} \cancel{\hspace{-1em}{\implies}\hspace{-1em}}\hspace{1em} b$a\hspace{1em}\cancel{\hspace{-1em}{\implies}\hspace{-1em}}\hspace{1em} b

7. $a\impliedby b$a\impliedby b

8. $a\cancel{\impliedby}b$a\cancel{\impliedby}b

9. $a\hspace{1em}\cancel{\hspace{-1em}{\impliedby}\hspace{-1em}}\hspace{1em} b$a\hspace{1em}\cancel{\hspace{-1em}{\impliedby}\hspace{-1em}}\hspace{1em}b

If one wants to use these in a post, it probably makes sense to use \newcommand at the start of your post:

\newcommand{\niff}{\hspace{1em}\cancel{\hspace{-1em}{\iff}\hspace{-1em}}\hspace{1em}}
\newcommand{\nimplies}{\hspace{1em}\cancel{\hspace{-1em}{\implies}\hspace{-1em}}\hspace{1em}}
\newcommand{\nimpliedby}{\hspace{1em}\cancel{\hspace{-1em}{\impliedby}\hspace{-1em}}\hspace{1em}}

After these definitions, a\niff b will produce $\newcommand{\niff}{\hspace{1em}\cancel{\hspace{-1em}{\iff}\hspace{-1em}}\hspace{1em}}a\niff b$, etc.

^{*In contrast with using negative space to overlay two symbols on top of each other; I do not know how to place the second symbol perfectly centered on top, in all browsers. In particular, nothing in this Answer looks good in my browser.}

Answer 3

$\def\nimplies{\implies\kern-1.8em/\kern1.3em}$ Seeing as the command \cancel from the package cancel has been recommended both in this question and in this earlier question on a similar subject, I repeat here the issue that I raised there: \cancel doesn't draw the stroke in the correct colour --- it appears to draw it in black, which is sometimes incorrect, especially when the background is black. Instead, how about defining a token to typeset the symbol you want crossed out (\iff or whatever), an appropriate negative \kern, / (the cancelling stroke) and an appropriate positive \kern to achieve the desired spacing at the right? For example, here's a way to use a crossed-out \implies symbol: first define a token \nimplies for that symbol, by $\def\nimplies{\implies\kern-1.8em/\kern1.3em}$, then you can write e.g. $P\nimplies Q$ for $P\nimplies Q$. Here's one I lined up against an \implies to check the spacing:

$\begin{gather} P\implies Q\\P\nimplies Q \end{gather}$

The stroke is about .05em to the left of the middle of the shaft, but I think it looks better slightly to the left, as the symbol's left end is less cluttered than its right end. If you want the stroke accurately centred, then $\def\nimplies{\implies\kern-1.75em/\kern1.25em}$.