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The English site has a question about "revoke" and "revocable", which includes the text

What's the reasoning for this? Are there any other examples like this?

I wanted to use that same text to ask a question about "$\σ2$" and "$\σ$". I first tried asking the question at https://math.stackexchange.com. The question was closed as off-topic on the Math site.

Why some calculations noted as "sym2" and "sym", while others noted as "symA" and "symB", where "symB" is the square root of "symA"?

Today I learnt that the standard deviation is calculated as square root of the mean of the squares of the deviations from the arithmetic mean of the distribution. The mean of the squares of the deviations from the arithmetic mean of the distribution is called the variance. The standard deviation is noted as $\σ$. However, there is no symbol for the variance. The variance is simply noted as $\σ2$. What is the reasoning for this? Are there any other examples like this?

Compare this with the Hilbert space. Look at the section which says "An element A of B(H) is called 'self-adjoint' or 'Hermitian' if A* = A. If A is Hermitian and ⟨$Ax, x⟩ \ge 0$ for every x, then A is called 'nonnegative', written $A \ge 0$; if equality holds only when x = $\theta$, then A is called 'positive'. The set of self adjoint operators admits a partial order, in which $A \ge B$ if $A − B \ge 0$. If A has the form B * B for some B, then A is nonnegative; if B is invertible, then A is positive. A converse is also true in the sense that, for a non-negative operator A, there exists a unique non-negative square root B such that $A = B2 = B * B$." Here, the non-negative operator is noted as A, and its non-negative square root is noted as B.

I really need to ask a question about "$\σ2$" and "$\σ$". What site can I ask such a question?

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What site do I ask a question about symbols?

If you are asking about the meaning of mathematical symbols (in a specific context), I suspect they are on topic on https://math.stackexchange.com/

If you are asking about the origin of mathematical symbols (as used in a specific context), they are on topic https://hsm.stackexchange.com

But you are basically asking why conventions for the use of mathematical symbols are inconsistent, in general and/or in specific cases. That is basically not objectively answerable:

  • In the general case, symbol usage is inconsistent because different mathematicians have their own ideas and preferences on how to express themselves. And there is no official standard on how mathematical symbols must be used.

  • In specific cases, the issue will be that different mathematicians have decided at different times to use different symbols for the same concept. They have published papers, and depending on how influential the papers were, some symbology has been been adopted by others.

(This is the same as for other languages. For example, English is replete with strange words with obscure origins and spellings. There is no process or standard for inventing new words. It just happens.)

Back to the topic of this question, since your question about "$\σ2$" and "$\σ$" has no objective answer, and no prospect of an objective answer, it is liable to be a magnet for "opinion-based" answers on just about every site where you might consider asking it. And that makes it off-topic on just about every Stack Exchange site.

In short: it is not the field that that makes your question off-topic. It is the kind of question that it is (i.e. not objectively answerable) that makes it off-topic.

See also: How many angels can dance on the head of a pin?


I really need to ask a question about "$\σ2$" and "$\σ$".

Umm ... I suspect that you are conflating "need" with "want". But either way, Stack Exchange is not a place to ask unanswerable questions.

You should try to find a suitable discussion forum ... or maybe the front bar at a statisticians' conference hotel.

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    I have right now tried at hsm.stackexchange.com/questions/14193/….
    – Arunabh
    Feb 15, 2022 at 21:12
  • I want to ask a question about "σ^2" and "σ" that is of the same type as revoke and revocable.
    – Arunabh
    Feb 15, 2022 at 21:28
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    I think you are not going to get any answer apart from 1) there is no "reasoning", and 2) there are lots of examples of inconsistent notation in mathematics. The key difference between this and questions about English word origins / spelling is that there sources that can answer the latter. (Also ... there are people who think that word origin questions are interesting in their own right.)
    – Stephen C
    Feb 15, 2022 at 23:47

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