Talk:Tensor

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Referring to orthonormal basis as frame

Latest comment: 1 month ago12 comments6 people in discussion

I see my edit changing "from one orthonormal basis (called a frame) to another" to "from one orthonormal basis to another" in the Spinors section was reverted without comment by Tito Omburo. Is there any need to refer to frames in the physics sense in this section? "Frames" have an entirely distinct linear algebra meaning (and this is primarily a linear algebra article). You could make the exact same point in fewer words and without the ambiguity by just saying "basis", right? — Preceding unsigned comment added by 2600:4040:2255:4300:7426:36C9:973E:3FC5 (talk) 16:04, 9 January 2025 (UTC)Reply

I reverted the edit because the section still uses the word frame throughout, but without clarifying that in this section "frame" means "orthonormal basis". Tito Omburo (talk) 17:38, 9 January 2025 (UTC)Reply

Yes, sorry, I missed that at first. This is my first Wikipedia edit. Would it be alright with you if I took a stab at rewriting the spinor section so as not to mention frames at all? 2600:4040:2254:3D00:3119:AB26:2A63:C265 (talk) 03:54, 10 January 2025 (UTC)Reply

What would you call the "space of frames"? Tito Omburo (talk) 10:21, 10 January 2025 (UTC)Reply

The more I think about this, I'm not even sure it makes sense to have two paragraphs about spinors on this page. We're essentially saying "Here's something that would transform like a tensor *if* orientation didn't matter which it *actually* does." But if you do think the tensor page should explain the basic motivation for spinors, perhaps something like: "Physical coordinate systems are often expressed in terms of orthonormal bases. Changes of orthonormal bases obtained from rotations transform the same way as tensors under rotation; however, not all possible orthonormal bases can be reached by rotation (because the orthogonal group representing all possible rotations has two connected components). Spinors and spin groups use a double cover of the orthogonal group to allow transformations between bases that can't be considered orientation-preserving rotations." 2600:4040:2257:3600:8546:6D3:38D8:37BD (talk) 19:34, 10 January 2025 (UTC)Reply

Darnit, that parenthetical should say "all changes of basis", not "all rotations". The whole point is that it includes things a simple rotation won't get you to. 2600:4040:2257:3600:8546:6D3:38D8:37BD (talk) 19:40, 10 January 2025 (UTC)Reply

Except this explanation is wrong. Spinors exist not because the orthogonal group is not connected, but because the special orthogonal group is not simply connected. Tito Omburo (talk) 19:44, 10 January 2025 (UTC)Reply

I believe you're mistaken. The orthogonal group has two connected components, one corresponding to orientation-preserving rotations (matrices with determinant 1) and one corresponding to orientation-flipping transformations (matrices with determinant -1). The special orthogonal group throws out the orientation-flipping ones and is simply connected. A quick Google search should lead you to multiple proofs of this fact. I was also wondering if there is a reason to explain spinors on this page at all. 2600:4040:2258:5B00:6C83:5698:CE47:BB6 (talk) 20:19, 10 January 2025 (UTC)Reply

No, the special orthogonal group is not simply connected. That's in some sense the whole point of spinors. Tito Omburo (talk) 20:57, 10 January 2025 (UTC)Reply

Yes, that is my understanding too. The special orthogonal group SO(n) has non-trivial fundamental group. Namely, its fundamental group is the group of two elements. The spin group is the two-to-one cover of SO(n) that corresponds to this fundamental group, according to covering space theory. Mgnbar (talk) 21:03, 10 January 2025 (UTC)Reply

It is also true that the orthogonal group O(n) has two connected components, one of which is the special orthogonal group SO(n). There is a two-to-one covering map from O(n) to SO(n). (In fact there are infinitely many such maps. I'm not aware of a canonical one.) But that's not what spin is about. Mgnbar (talk) 21:03, 10 January 2025 (UTC)Reply

@Tito Omburo and Mgnbar:This really belongs in Tensor fields A basis pertains to a vector space, while a frame pertains to a coordinate patch in a differentiable manifold . The physics literature often refers to spinor fields as spinors, tensor fields as tensors and vector fields as vectors, but this article is supposed to be about the algebraic entities, not the corresponding fiber bundles on manifolds.. -- Shmuel (Seymour J.) Metz Username:Chatul (talk)

Well, I've seen "frame" refer either to a basis of the tangent space at a point or a section of the frame bundle, in both mathematical and physical contexts. In particular, Cartan famously studied "moving frames", which wouldn't be a sensible combination of words unless a frame was fixed and not a field. The main thing is consistency in usage in the article, and if, instead of saying "orthonormal basis", we can get away with using the simpler term "frame", which also borrows from the physical idea "frame of reference" that we are trying to make accessible, all the better. (Incidentally, I would use the term "frame field" to refer to the field case. See, e.g. Michor Topics in differential geometry. ) Tito Omburo (talk) 12:00, 14 July 2025 (UTC)Reply

The bulk of this article should not use the Einstein summation convention

Latest comment: 1 year ago2 comments2 people in discussion

The bulk of this article should not use the Einstein summation convention. The Einstein summation convention should be introduced in a special section near the bottom of the article. 134.16.68.202 (talk) 01:22, 25 January 2024 (UTC)Reply

I tend to agree, although I don't feel strongly. There are a few cases (general transformation rule) where explicitly showing the summations would significantly expand the length of the expression. But it would be a net benefit to novice readers. Mgnbar (talk) 21:07, 25 January 2024 (UTC)Reply

Incorrect definition

Latest comment: 1 year ago2 comments2 people in discussion

The definitions in the "as multilinear maps" and "using tensor products" sections are wrong. Requiring all copies of to be the same vector space would exclude non-square matrices from being tensors (a ${\displaystyle 2\times 3}$  matrix for example is a multilinear map ${\displaystyle \mathbb {R} ^{2}\times \mathbb {R} ^{3}\to \mathbb {R} }$ ). 65.93.245.82 (talk) 06:09, 23 April 2024 (UTC)Reply

The definition is not so much incorrect as restrictive. This article views tensors in terms of a single vector space V and its dual V*. In the past, we have had discussions about whether "tensor" simply means "element of a tensor product of vector spaces", which would be broad enough to handle your objection. The consensus was that the term "tensor" is not used that broadly in the literature. Mgnbar (talk) 11:46, 23 April 2024 (UTC)Reply

First image and caption don't match

Latest comment: 1 year ago6 comments5 people in discussion

The image at the top of the article and its caption are mismatched in several fairly confusing ways. First, the unit vectors e are not shown in the picture. Second, the output vector T is not shown in the picture. The reader is left trying to guess which e goes where, and why there are three output vectors when it says a second-order tensor has only one. Also, the σ coefficients aren't mentioned in the caption at all. 2600:8800:1180:25:AD40:E4C1:631E:5E07 (talk) 21:58, 25 April 2024 (UTC)Reply

Surely the ${\displaystyle e_{i}}$  are represented by the locations at which ${\displaystyle T}$  is evaluated, the outputs ${\displaystyle T(e_{i})}$  are shown in a bluish color? --JBL (talk) 23:26, 25 April 2024 (UTC)Reply

The anonymous poster has a point. One can guess where the ${\displaystyle e_{i}}$  are, but one should not have to. More importantly, ${\displaystyle \sigma }$  is prominent in the figure and missing from the caption. In fact, is it true that ${\displaystyle \sigma =T}$ ? Or is the figure reserving ${\displaystyle T}$  for the traction vectors that the stress tensor ${\displaystyle \sigma }$  produces? If that's the case, then is the caption wrong? It's all a bit of a mess. Mgnbar (talk) 00:22, 26 April 2024 (UTC)Reply

It is helpful to compare with an earlier revision revision of the caption, although this doesn't fully resolve the difficulty. Tito Omburo (talk) 13:14, 26 April 2024 (UTC)Reply

It doesn't help that everything is a subscript, preventing use of the Einstein summation convention. I would guess that ${\displaystyle {\overrightarrow {\sigma _{ij}}}=\mathbf {T} ({\overrightarrow {e_{i}}})\cdot {\overrightarrow {e_{j}}}{\overrightarrow {e_{j}}}}$ , the projection of ${\displaystyle \mathbf {T} ({\overrightarrow {e_{i}}})}$  on ${\displaystyle {\overrightarrow {e_{j}}}}$ . Things would have been clearer had the caption spelled things out, used ${\displaystyle \mathbf {T} =T^{ij}{\overrightarrow {e_{i}}}{\overrightarrow {e_{j}}}}$  and not introduced an apparently extraneous additional name. I agree with Tito Omburo that the earlier version is clearer, although I find the nomenclature awkward. -- Shmuel (Seymour J.) Metz Username:Chatul (talk) 13:33, 26 April 2024 (UTC) -- Revised 19:04, 26 April 2024 (UTC)Reply

The current caption does describe what/where the e_i are (they are the normals to the faces of the cube). I agree about the sigmas (I mean, I know what must have been intended, but it's not good that it's entirely implicit). --JBL (talk) 18:16, 26 April 2024 (UTC)Reply

Machine learning still unclear

Latest comment: 1 month ago8 comments3 people in discussion

@Tito Omburo: A recent edit, permalink/1300282879, removed a {{clarify|text=the dimension of the spaces along the different axes}} template without actually clarifying the text in question. An axis is indexed by a scalar, e.g., real number, complex number. Changing axis to slot without defining slots leaves the dimension of the spaces belonging to the different slots just as murky as before. -- Shmuel (Seymour J.) Metz Username:Chatul (talk) 11:57, 15 July 2025 (UTC)Reply

I never understood the clarification that was offered. Can we please discuss it relative to the mathematical meaning of tensor?

Suppose that we have an n-dimensional vector space V, and we choose a basis for it. Then a linear transformation from V to itself is an element of ${\displaystyle V^{*}\otimes V}$ , and it can be written as an nxn matrix M. The entries of M are denoted M_ij (or maybe one of the indices is superscripted).

Are the indices i and j indexing the "axes"/"slots", one of which is (for) ${\displaystyle V^{*}}$  and one of which is (for) ${\displaystyle V}$ ? Is that the meaning of "axis"/"slot"?

If so, then an axis is indexed by an integer between 1 and n — not by a general real or complex number. Please help me understand. Mgnbar (talk) 12:53, 15 July 2025 (UTC)Reply

Actually in machine learning, a tensor is just a multidimensional array, the axes are the different dimensions of the tensor (just like a rectangular matrix has horizontal and vertical axes.) Thus a tensor belongs to ${\displaystyle V_{1}\otimes V_{2}\otimes \dots \otimes V_{k}}$  where each factor is a module over a "ring" (of floating point numbers, usually), each having a fixed basis. The axes refer to the ${\displaystyle V_{i}}$ , and they generally have different dimensions. Tito Omburo (talk) 13:01, 15 July 2025 (UTC)Reply

I think that your description agrees with mine (where they overlap). But then I don't understand how Chatul says that the axes are indexed by real or complex numbers. Mgnbar (talk) 14:38, 15 July 2025 (UTC)Reply

I misread the text. Not enough coffee. :-(

BTW, are Tito Omburo's scare quotes around ring because FP arithmetic violates the distributive and associative properies? -- Shmuel (Seymour J.) Metz Username:Chatul (talk) 15:23, 15 July 2025 (UTC)Reply

Indeed! Tito Omburo (talk) 16:58, 15 July 2025 (UTC)Reply

Thanks for clarifying, both of you. Happy travels. Mgnbar (talk) 17:10, 15 July 2025 (UTC)Reply

Thanks everyone, from me as well. Always good to make progress! Tito Omburo (talk) 19:11, 15 July 2025 (UTC)Reply