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    • CommentRowNumber1.
    • CommentAuthorDavid_Corfield
    • CommentTimeDec 11th 2018

    Will start something here.

    v1, current

    • CommentRowNumber2.
    • CommentAuthorDavid_Corfield
    • CommentTimeDec 11th 2018

    Some content

    v1, current

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeDec 11th 2018

    Thanks!. I slightly edited the first sentence for links:

    We do have Euclidean group, albeit just a stub.

    And, due to the dominance of logicians around here, for better or worse our lattice points to the order-theoretic concept, while for the “common” meaning we need to type lattice in a vector space.

    diff, v2, current

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeFeb 26th 2019

    expanded the Idea/Definition-section, in particular added this diagram:

    1 1 normal subgrouplattice of translations N E translationgroup crystallographicgroup S Iso(E) Euclideanisometry group pointgroup G O(E) orthogonalgroup 1 1 \array{ & 1 && 1 \\ & \downarrow && \downarrow \\ {\text{normal subgroup} \atop \text{lattice of translations}} & N &\subset& E & {\text{translation} \atop \text{group}} \\ & \big\downarrow && \big\downarrow \\ {\text{crystallographic} \atop \text{group}} & S &\subset& Iso(E) & {\text{Euclidean} \atop \text{isometry group}} \\ & \big\downarrow && \big\downarrow \\ {\text{point} \atop \text{group}} & G &\subset& O(E) & {\text{orthogonal} \atop \text{group}} \\ & \downarrow && \downarrow \\ & 1 && 1 }

    Then I added the remark (here) that the normality of the subgroup NN is what makes the action of the point group GG descend to the torus E/NE/N.

    Is there a good name for these tori E/NE/N equpped with actions of Euclidean point groups GG? They want to be called “representation tori” to go along with representation spheres, but I guess nobody says that.

    diff, v3, current

    • CommentRowNumber5.
    • CommentAuthorUrs
    • CommentTimeFeb 26th 2019
    • (edited Feb 26th 2019)

    started to add various references.

    Added a long quote from

    • E. V. Chuprunov, T. S. Kuntsevich, nn-Dimensional space groups and regular point systems, Comput. Math. Applic. Vol. 16, No. 5-8, pp. 537-543, 1988 (doi:10.1016/0898-1221(88)90243-X)

    on the history and current scope of the classification.

    Not done yet, but need to interrupt now.

    diff, v4, current

    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeAug 31st 2019
    • (edited Aug 31st 2019)

    added pointer to

    • D. Weigel, T. Phan and R. Veysseyre, Crystallography, geometry and physics in higher dimensions. III. Geometrical symbols for the 227 crystallographic point groups in four-dimensional space, Acta Cryst. (1987). A43, 294-304 (doi:10.1107/S0108767387099367)

    am trying to find out which of the finite subgroups of SU(2) in the D- and E-series arise, via their canonical action on 2 4\mathbb{C}^2 \simeq_{\mathbb{R}} \mathbb{R}^4, as point groups of crystallographic groups in 4d.

    diff, v5, current

    • CommentRowNumber7.
    • CommentAuthorDavid_Corfield
    • CommentTimeAug 31st 2019

    Is it just a matter of going through a list? The list in ’Summary’ tells you which are non-crystallographic.

    • CommentRowNumber8.
    • CommentAuthorUrs
    • CommentTimeAug 31st 2019

    Yes, it should be. But I have trouble parsing the entries.

    Does D nD_n denote the dihedral group? Does the binary dihedral group appear anywhere?

    • CommentRowNumber9.
    • CommentAuthorUrs
    • CommentTimeAug 31st 2019

    So I suppose I can easily check by direct inspection that the following are crystallographics groups NGN \rtimes G in 4d, with N= 4N = \mathbb{Z}^4 the canonical translational subgroup of 4\mathbb{R}^4 and the point group GG acting on 4 \mathbb{R}^4\simeq_{\mathbb{R}} \mathbb{H} via their inclusion as subgroups of Sp(1)Sp(1):

    NG=...N \rtimes G = ...

    1. 4 2\mathbb{Z}^4 \rtimes \mathbb{Z}_2

    2. 4 4\mathbb{Z}^4 \rtimes \mathbb{Z}_4

    3. 4Q 8\mathbb{Z}^4 \rtimes Q_8

    So these must appear somewhere in these lists. But where? I must be missing something.

    • CommentRowNumber10.
    • CommentAuthorDavid_Corfield
    • CommentTimeSep 1st 2019

    The term ’binary dihedral group’ shows up on the 3d page, but not given in Coxeter notation. Coxeter notation seems confusing.

    • CommentRowNumber11.
    • CommentAuthorGuest
    • CommentTimeJan 18th 2020

    First, great site. It’s been very useful to me in many situations.

    The definition of the point group as the quotient S/N seems to me to be inequivalent to the standard definition (see Wiki for ex), because the point group usually is the the biggest subgroup of S to keep one point fixed, which, if I understand correctly, is only true if the associated short exact sequence splits.

    In the case of a non-symmorphic space group, S might include screw-axis transformations and glide reflections, which can contain translations of fractional lattice vectors, hence should remain in the quotient. I could be missing something, I am fairly new to this topic.

    Best regards,
    Sami Siraj-Dine.
    • CommentRowNumber12.
    • CommentAuthorUrs
    • CommentTimeJan 18th 2020

    Thanks for your comment. If there is an error I’ll want to fix it.

    Let me see…

    Hm, checking again, it looks to me like the statement in the entry agrees with the terminology associated with Bieberbach’s first theorem; fully explicitly so in the way it is stated in the recent arXiv:1907.02021, Theorem 2.3 and paragraph below (on p. 4).

    Also arXiv:1208.5055 seems to agree, less authoritatively but all the more seemingly regarding it as standard: around (0.2) on p. 3.

    Finally, I wouldn’t think that what you mention regarding point groups possibly containing screw-axis transformaions etc. is in contradiction to this statement. I suppose the “fractional” in your “fractional lattice vectors” signifies this.

    But if you think I am missing your point, please say so and let’s try to sort this out.

    • CommentRowNumber13.
    • CommentAuthorGuest
    • CommentTimeJan 19th 2020

    Thank you for your answer!

    I’m sorry, I am not sure I understand you properly. I tried reading the references you provide, and they support the definition you provide on the site, but I am afraid I am not able to wrap my head around the implications of requiring NN to be a free normal abelian subgroup (mostly whether “free” is a constraining hypothesis or not). But the question I am asking fortunately should not need this concept, if I got things right.

    Let me rephrase my previous post to be sure we agree on the question, and that I am not making a mistake in my thinking.

    • Taking a non-symmorphic space group SS means that there is a symmetry operation (R|a)(R|a) that contains both an orthogonal RR and a translation part t a:xx+at_a : x \mapsto x+a, where the translation t at_a does not belong to the lattice NN (in a simple counter-example I saw, aa was half of a lattice vector, and that’s what I meant by fractional, it was not very clear, my bad).

    • Hence, the quotient S/NS/N would have an equivalence class corresponding to (R|a)(R|a), which is not an element of the orthogonal group. Is that correct?

    • That seems to be in contradiction with the definition of point groups I’ve read in crystallography physics books which requires them to be a subgroup of the orthogonal group, and to the inclusion that is written in the short exact sequences on the page.

    It is probably a misunderstanding on my part, apologies if that is the case.

    Best regards, Sami Siraj-Dine

    • CommentRowNumber14.
    • CommentAuthorUrs
    • CommentTimeJan 19th 2020

    Thanks for your reply. Now I see more clearly what you are saying.

    First regarding the “free”-ness condition in the normal subgroup: That’s just part of the very definition of what a discrete translation group should be. It just means that the translation group is isomorphic to n\mathbb{Z}^n, and that’s what we want by definition when speaking about crystals.

    Regarding your worries about the quotient: While I don’t know off the top of my head if the quotient of a crystallographic group by any normal translations subgroup is always guaranteed to be a subgroup of the orthogonal groups (you seem to suspect it is not), this is also not what is being claimed in the entry, is it. In the entry we demand explicitly the horizontal inclusions shown in the diagram reproduced now:

    1 1 normal subgrouplattice of translations N E translationgroup crystallographicgroup S Iso(E) Euclideanisometry group pointgroup G O(E) orthogonalgroup 1 1 \array{ & 1 && 1 \\ & \downarrow && \downarrow \\ {\text{normal subgroup} \atop \text{lattice of translations}} & N &\subset& E & {\text{translation} \atop \text{group}} \\ & \big\downarrow && \big\downarrow \\ {\text{crystallographic} \atop \text{group}} & S &\subset& Iso(E) & {\text{Euclidean} \atop \text{isometry group}} \\ & \big\downarrow && \big\downarrow \\ {\text{point} \atop \text{group}} & G &\subset& O(E) & {\text{orthogonal} \atop \text{group}} \\ & \downarrow && \downarrow \\ & 1 && 1 }

    Or maybe that point needs to be stated better? If that’s what it is, I’d be happy to try to improve the text in this regard.

    • CommentRowNumber15.
    • CommentAuthorGuest
    • CommentTimeJan 19th 2020
    Ah, the diagram is a condition, not a statement! Indeed, that was the source of my confusion.

    Thanks a lot for clarifying that!

    Sami Siraj-Dine
    • CommentRowNumber16.
    • CommentAuthorUrs
    • CommentTimeJan 19th 2020

    Okay, thanks for saying. True, the text didn’t say this well. I have now adjusted the text in the entry to bring this out more properly.

    diff, v8, current

    • CommentRowNumber17.
    • CommentAuthorJohn Baez
    • CommentTimeMay 2nd 2020

    It’s nice to see a mathematically precise treatment of space groups, aka crystallographic groups. But I don’t think this can be right:

    Equivalently, a crystallographic group on a Euclidean space EE is a finite subgroup SIso(E)S \subset Iso(E) of the isometry group of EE (its Euclidean group) that contains a lattice NEIso(E)N \subset E \subset Iso(E) of translations as a normal subgroup NSN \subset S, such that the corresponding quotient group, called the point group of the crystallographic group, is a subgroup GS/NO(E)G \coloneqq S/N \;\subset\; O(E) of the orthogonal group.

    How can EE be finite and contain a lattice of translations? Besides, crystallographic groups just aren’t finite.

    I would just delete “finite” but I’m thinking there should be some other word here. So, I’ll just leave a warning.

    diff, v9, current

    • CommentRowNumber18.
    • CommentAuthorTodd_Trimble
    • CommentTimeMay 2nd 2020

    Perhaps a semidirect product of the lattice translation group (as normal subgroup) with a finite point group generated by reflections? Just guessing.

    • CommentRowNumber19.
    • CommentAuthorUrs
    • CommentTimeMay 3rd 2020

    Thanks for catching. This should have been “discrete” of course, – have fixed it now.

    diff, v10, current

    • CommentRowNumber20.
    • CommentAuthorUrs
    • CommentTimeMay 3rd 2020
    • (edited May 3rd 2020)

    Not sure if in addition we need to require the quotient group (the point group) to be finite, it feels like this should follow.(?)

    • CommentRowNumber21.
    • CommentAuthorTodd_Trimble
    • CommentTimeMay 3rd 2020

    If I follow, it should follow: compact + discrete = finite.

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