MathJax on CodeReview

Question

Several times I had the need to write mathematical formulas in my answers on CodeReview.SE. Sometimes, this can be simulated by sup and/or sub HTML tags, as I realized when syb0rg edited my answer here. However, sometimes it is not as easy, for example here, where I had to use code block for one formula (to achieve alignment) and LaTeX formulas off-site and attach them as images (for summations).

Formulas make sense in the design of various (for example, numerical) algorithms, as well as in the analysis of the algorithm's complexity.

My suggestion is to include MathJax in CodeReview.SE, as it was included in Math.SE (where it obviously makes even more sense to be included), unless there is some big drawback here that I'm failing to notice.

Answer 1

Enough! Enough! If you post any more answers here, we're going to run out of room in the database!

Er, I mean, you've done a good job of presenting evidence, and have convinced us on the Community Team. As of about ten minutes ago, MathJax has been enabled for Code Review. Use it well!

Answer 2

Based on Space Exploration SE's similar feature request, the SE devs would want to see a list of questions that can benefit from this feature. We may not use this feature a whole lot, but we at least need several good examples of posts that would benefit from this. Even then, they may not feel that it's a good fit for CR, but that's something they will have to determine based on whatever criteria they use.

Answer 3

The following sites support MathJax (there may be more I'm unaware of):

• Math Overflow
• Math
• Cross Validated
• Theoretical Computer Science
• Computer Science
• Computational Science
• Cryptography
• Signal Processing
• Physics
• Chemistry
• Biology
• Space Exploration

If even Biology supports MathJax, then leaving MathJax disabled on Code Review looks more like an oversight than a deliberate rejection. Let's enable it!

(Interestingly, TeX — LaTeX does not support MathJax.)

Answer 4

This answer to the Knight's Distance required taking a 'snip' screenshot to get the formula in.

Screen-Shot Snip

Answer 5

In Optimizing unique partitions of integers, I just want to write the quadratic equation nicely. Having to resort to ASCII art to do so is just ridiculously frustrating!

              _________
        -1 ± √ 1 + 8 n
x    = —————————————————
 max          2

Have I mentioned the mad keyboard skills necessary to type symbols ± √ —?

The lazy workaround would be to write

x = (-1 + sqrt(1 + 8 * n)) / 2

but that just doesn't make the answer nearly as easy to understand.

Also, getting the equals signs to line up in

n = 1 + 2 + 3 + … + x_max

    = x_max (x_max + 1) / 2

was no mean feat!

Answer 6

Here's an answer that would have been a lot easier to write if MathJax had been available:

• Is there any more optimal way to solve this idempotent equation?

  So if n can be factored as p₁^e₁ . p₂^e₂ . ... then each p_i^e_i must divide either a or a − 1 (because each p_i can divide into at most one of a and a − 1).

Answer 7

This answer could have been a lot less awkward if proper mathematical notation were a reasonable option.

You should find an algorithm finding the root square of 1 module n.

x^2 = 1 mod n

To do that:

• Factorize n in prime factor. n = p1^e1 * p2^e2 * ... * pk^ek
• Find the root for all prime xi^2 = 1 mod pi^ei. easy, it is 1 and -1 (or pi - 1)
• Then use the Chinese theorem to find the solution for x (x = x1 mod x1^e1, x = ..., x = xk mod pk^ek (xi can take 2 value so you will have k^2 answer)

you just need to take the solution which meet your condition

I don't blame the poster for the poor typesetting. It's an unreasonable burden to have to type

_n_ = _p_<sub>1</sub><sup>_e_<sub>1</sub></sup> ⋅ _p_<sub>2</sub><sup>_e_<sub>2</sub></sup> ⋅ … ⋅ _p_<sub>_k_</sub><sup>_e_<sub>_k_</sub></sup>

to obtain

n = p₁^e₁ ⋅ p₂^e₂ ⋅ … ⋅ p_k^e_k

Answer 8

This answer was a pain to type without MathJax, with all the italics, subscripts, and the Greek alpha character:

S_t = α Y_t-1 + (1 - α) S_t-1

where

• α is the decay rate
• Y_t is the value at time t
• S_t is the exponential moving average at time t.

That equation was taken from the linked Wikipedia article, so I wanted to copy it faithfully into the answer for discussion.

Answer 9

Even using subscripts and superscripts, formulas still look odd and can be hard to read. An example would be from this answer:

Is it reasonable to assume that most numbers will be non-palindromes? If not, I would suggest something like this:

1. Determine the approximate number of digits in your number a using mpz_sizeinbase() and call this number n.

2. Compute a / d^(n-30), then compute a % d³⁰. Print both in base d and compare. If they are unequal, reject as non-palindrome.

   Computing a / d^(n-30) will be fast using mpz_tdiv_q(). Unfortunately, computing d³⁰ is not fast. Perhaps a rough approximation of that would also increase efficiency.

MathJax would make that a lot easier to read IMO.

Answer 10

Answers which use Landau-notation benefit from MathJax as well, since it is often necessary to perform algebraic manipulations to reach the final expression for the asymptotic complexity.

For example, when discussing optimal search implementations.

Answer 11

If MathJax were available, the author of this question might have been able to describe what the code was trying to accomplish:

• Improve performance of math function applied to arrays

Also, if a reviewer chooses to overhaul the code, it would also be helpful to write an explanation using MathJax.

Answer 12

Since we're giving the usage examples to show how much is this needed, here are those linked in the question:

Here, I had to use code block for one formula (to achieve alignment) and LaTeX formulas off-site and attach them as images (for summations).

Here are the formulas:

    sum = (a2 - a1) + (a3 - a1) + (a4 - a1) + ... + (an - a1) +
                    + (a3 - a2) + (a4 - a2) + ... + (an - a2) +
                                + (a4 - a3) + ... + (an - a3) +
                                            ...

Answer 13

In Counting occurrences of values in C Array (Shannon Entropy), the entropy formula was posted as an image.

A MathJax expression would have been better.

Answer 14

In this answer, the author took the effort to write

sqrt(⌈log₁₀(B)⌉*81)

The subscript takes a little bit of effort. The brackets for the ceiling function aren't easy to type at all. Then there's still the sqrt(…) which is impossible to nicely typeset using regular Markdown.