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I am wondering which research communities use the notation $\mathbf i$, $\mathbf j$, $\mathbf k$ for the three-dimensional unit vectors. The calculus textbook I have to use (Stewart) uses that notation. But I am a mathematician, so maybe there is a physics or engineering community out there where that notation is actually used.

Notations that I am well acquainted with include

  • $e_1$, $e_2$, $e_3$
  • $e_x$, $e_y$, $e_z$
  • $\mathbf e_1$, $\mathbf e_2$, $\mathbf e_3$
  • $\mathbf e_x$, $\mathbf e_y$, $\mathbf e_z$
  • ...

I have never seen $\mathbf i$, $\mathbf j$, $\mathbf k$ in any research paper ever. Can you point out a recent example?

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    $\begingroup$ This question doesn't seem to be about mathematics education since you're not interested in textbooks that use the notation. $\endgroup$
    – JRN
    Commented Mar 4, 2018 at 11:12
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    $\begingroup$ It is about mathematics education. I am interested in a textbook that uses that notation since I have to use it. I would like to understand what I am teaching to my students. $\endgroup$
    – shuhalo
    Commented Mar 4, 2018 at 15:51
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    $\begingroup$ In my opinion the question asks for the use of these symbols in research papers, which takes this outside the context of mathematics education. If you want to know about the use of this notation in textbooks (other than Stewart) that would seem like an on-topic question. $\endgroup$
    – mweiss
    Commented Mar 4, 2018 at 20:28
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    $\begingroup$ "I would like to understand" -- surely the problem isn't that you don't understand the notation. You just want to know whether that notation is common to particular communities. $\endgroup$
    – mweiss
    Commented Mar 4, 2018 at 20:29
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    $\begingroup$ I'm voting to close this question as it asks about the use of a specific notation in research, which is (just slightly) off-topic for this site. It could easily be edited to bring it on-topic, for example by asking about textbooks rather than research papers. $\endgroup$
    – mweiss
    Commented Mar 5, 2018 at 2:18

4 Answers 4

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Search arxiv.org for "unit vectors i, j, k" and you will find examples of research papers where the notation is used. Many are from physics communities investigating phenomena in three-dimensional space. Clearly we should not expect to find this notation in research relating to general dimensionality.

In the image see "unit vectors i, j, k" at the bottom.

Search Field on arXiv

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    $\begingroup$ Can you please give me one such example? I really don't see it and would like to know. Thanks. $\endgroup$
    – shuhalo
    Commented Mar 4, 2018 at 17:57
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    $\begingroup$ I don't think this is the place to include links to arXiv papers, but in my answer I placed a screen shot so you can see how to do the search. Pick some hits and search the pdf for "unit vector" and you will find examples, If you reorder by date, the most recent paper is titled "Lorentzian geometry of qubit entanglement." $\endgroup$
    – user52817
    Commented Mar 4, 2018 at 18:42
  • $\begingroup$ It might be of note that using the same procedure for "unit quaternions i j k" brings up papers in stable homotopy theory, differential geometry, and mathematical physics. $\endgroup$
    – K B Dave
    Commented Mar 4, 2018 at 20:54
  • $\begingroup$ I only tried google scholar before but the arxiv searched indeed brought up a few examples. Good point. $\endgroup$
    – shuhalo
    Commented Mar 5, 2018 at 2:06
  • $\begingroup$ Three years later, that search page is completely changed and the equivalent search does not return relevant results anymore. So it would have been a good idea to include those links directly in the first place... $\endgroup$
    – Federico Poloni
    Commented Feb 25, 2021 at 17:31
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I'm not sure this a research setting, but the cross product relations $i \times j = k$, $j \times k = i$, etc are nice for illustrating the connection between quaternions (where $ij=k, jk=i$, etc) and the cross product.

Specifically, the Lie group consisting of unit length quaternions has Lie algebra $\mathbb{R}^3$ with Lie bracket given by cross product.

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  • $\begingroup$ Isn't this a bit confusing in this context? The quaternions form a four-dimensional space with basis $1,i,j,k$. The subspace spanned by $i,j,k$ (pure quaternions) is indeed three-dimensional, but not a subalgebra. $\endgroup$
    – Michał Miśkiewicz
    Commented Feb 8, 2021 at 12:46
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This notation is completely standard in all Physics classes at the undergraduate level. I doubt you would find any Physicist anywhere who does not instantly recognize that notation -- but then, I would also have said the same thing about mathematicians, as the notation is completely standard in multivariable calculus at the undergraduate level in the United States.

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  • $\begingroup$ It is completely unheard of in Europe, and I haven't seen it in mathematics from graduate level onwards. So it was natural to ask where the notation is used in practice. $\endgroup$
    – shuhalo
    Commented Mar 5, 2018 at 4:42
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    $\begingroup$ It's certainly in widespread use in the UK. $\endgroup$
    – mweiss
    Commented Mar 5, 2018 at 4:43
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    $\begingroup$ It's certainly quite common in Italy $\endgroup$
    – Nicola Ciccoli
    Commented Mar 6, 2018 at 9:04
  • $\begingroup$ The question asks which "research communities" use this notation. While it is certainly true that this notation is standard in multivariable calculus classes in the US, it is equally true that not so many mathematicians use it in their research papers. $\endgroup$
    – Dan Fox
    Commented Mar 9, 2018 at 6:32
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Who?

Crystallographers sometimes in my experience. But, first page of a Google Scholar search gave 9/10 results from data science statistics, PCA and such. Some in an application of statistics like a specific medical paper, others just stats or computing in general). There was only one paper that was crystal-related (lattice vibrations, but in a physics context, not structure determination). So presumably space vectors or lattice vectors (perhaps not Cartesian).

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C47&q=i+j+k+vectors&btnG=

(You may see slightly different specific results from your search, Google being Google, but no matter, still shows you some people using ijk)

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  • $\begingroup$ Can you give me an example paper? The results that I see use ijk mostly as indices, or maybe I am just blind, or Google is deliberately fooling me. $\endgroup$
    – shuhalo
    Commented Mar 4, 2018 at 15:49
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    $\begingroup$ I think you are right. My bad. $\endgroup$
    – guest
    Commented Mar 4, 2018 at 16:19

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