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Let K be a global field, that is, a number field or a global function field. It is known that the answer to the question in the title over K is "Yes" when K has no real embeddings. We show that otherwise the answer is "No". Namely, we show that when K is a number field admitting a real embedding, it is impossible to define a group structure on the first Galois cohomology sets H^1(K,G) for all reductive K-groups G in a functorial way., Comment: 5 pages. This is a part of arXiv:2403.07659 to be published separately

Using ideas and results of Sansuc, Colliot-Th\'el\`ene, and Tate, we compute the defect of weak approximation for a reductive group G over a global field K in terms of the algebraic fundamental group of G., Comment: v1: 12 pages; v2: 13 pages; v3: 15 pages; v4: 14 pages

For a connected reductive group $G$ over a local or global field $K$, we define a diamond (or power) operation $$(\xi,n)\mapsto \xi^{\Diamond n}\,\colon\, H^1(K,G)\times {\mathbb Z}\to H^1(K,G)$$ of raising to power $n$ in the Galois cohomology pointed set (this operation is new when $K$ is a number field). We show that this power operation has many functorial properties. When $G$ is a torus, the set $H^1(K,G)$ has a natural group structure, and $\xi^{\Diamond n}$ then coincides with the $n$-th power of $\xi$ in this group. Using this power operation, for a cohomology class $\xi$ in $H^1(K,G)$ over local or global field, we define the period ${\rm per}(\xi)$ to be the least integer $n\ge 1$ such that $\xi^{\Diamond n}=1$. We define the index ${\rm ind}(\xi)$ to be the greatest common divisor of the degrees $[L:K]$ of finite extensions $L/K$ splitting $\xi$. The period and index of a cohomology class generalize the period and index a central simple algebra over $K$. For any connected reductive group $G$ defined over a local or global field $K$, we show that ${\rm per}(\xi)$ divides ${\rm ind}(\xi)$ and that ${\rm per}(\xi)$ and ${\rm ind}(\xi)$ always have the same prime factors., Comment: V.1: 52 pages. V.3: 61 pages, with an apendix by Zev Rosengarten. V4: 37 pages. Some parts have been moves to two other preprints

We classify the non-degenerate two-step nilpotent Lie algebras of dimension 8 over the field of real numbers, using known results over complex numbers. We write explicit structure constants for these real Lie algebras., Comment: 21 pages

Let G be a linear algebraic group, not necessarily connected or reductive, over the field of real numbers R. We describe a method, implemented on computer, to find the first Galois cohomology set H^1(R,G). The output is a list of 1-cocycles in G. Moreover, we have an implemented algorithm that, given a 1-cocycle z in Z^1(R,G), finds the cocycle in the computed list to which z is equivalent, together with an element of G(C) realizing the equivalence., Comment: V1: 41 pages. V2: 45 pages, the final version to appear in J. London Math. Soc

Generalizing Tate's results for tori, we give closed formulas for the abelian Galois cohomology groups H^1_{ab}(F,G) and H^2_{ab}(F,G) of a connected reductive group G over a global field F, and obtain formulas, suitable for computer calculatios, for the first nonabelian Galois cohomology set H^1(F,G) of G and for the second Galois cohomology group H^2(F,T) of an F-torus T., Comment: V1,2: 43 pages. V3: 58 pages. V4: 88 pages. V5: 101 pages, v6 :109 pages

Let $G$ be a connected reductive group over a number field $F$, and let $S$ be a set (finite or infinite) of places of $F$. We give a necessary and sufficient condition for the surjectivity of the localization map from $H^1(F,G)$ to the "direct sum" of the sets $H^1(F_v,G)$ where $v$ runs over $S$. In the appendices, we give a new construction of the abelian Galois cohomology of a reductive group over a field of arbitrary characteristic., Comment: V.1: 10 pages. V.2: 15 pages, with two new appendices, one by Borovoi and another one by Rosengarten. V.3: 15 pages, final version, to appear in Comptes Rendus Mathematique

Let $X$ be a homogeneous space of a connected linear algebraic group $G$ defined over the field of complex numbers $\mathbb C$. Let $x\in X({\mathbb C})$ be a point. We denote by $H$ the stabilizer of $x$ in $G$. When $H$ is connected, we compute the topological fundamental group $\pi_1^{\rm top}(X({\mathbb C}),x)$. Moreover, we compute the second homotopy group $\pi_2^{\rm top}(X({\mathbb C}),x)$., Comment: V.1: 10 pages. V.2: 13 pages. V.3: 13 pages. This corresponds to the final version to appear in J. Lie Theory, except that we have added a reference to a very recent preprint of Popov

Any complex-analytic vector bundle $\mathbb E$ admits naturally defined homotheties $\phi_{\alpha}$, $\alpha\in \mathbb C^*$, i.e. $\phi_{\alpha}$ is the multiplication of a local section by a complex number $\alpha$. We investigate the question when such automorphisms can be lifted to a non-split supermanifold corresponding to $\mathbb E$. Further, we compute the automorphism supergroup of a $\Pi$-symmetric super-Grassmannian $\Pi\!\operatorname{Gr}_{n,k}$, and, using Galois cohomology, we classify the real structures on $\Pi\!\operatorname{Gr}_{n,k}$ and compute the corresponding supermanifolds of real points., Comment: 42 Pages, appendix written by Mikhail Borovoi. A missed case fixed, the paper has been significantly revised

Using results of Cartan, Matsumoto, and Casselman, we give a short proof of Timashev's theorem computing the real component group \pi_0 G(R) of a connected reductive real algebraic group G in terms of a maximal torus of G containing a maximal split torus., Comment: 4 pages

Let G be a connected reductive group over the field of real numbers R. Using results of our previous joint paper, we compute combinatorially the first Galois cohomology set H^1(R,G) in terms of reductive Kac labelings. Moreover, we compute the group of connected components \pi_0 G(R) of the real Lie group G(R) and the maps in exact sequences containing \pi_0 G(R) and H^1(R,G)., Comment: V.1: 35 pages, v.2: 41 pages, v.3 41 pages

We introduce the notion of a quasi-connected reductive group over an arbitrary field to be an almost direct product of a connected semisimple group and a quasi-torus (a smooth group of multiplicative type). We show that a linear algebraic group is quasi-connected reductive if and only if it is isomorphic to a smooth normal subgroup of a connected reductive group., Comment: Withdrawn because it is superseded by the version 2 of arXiv:2103.04654 [math.RT] by the same authors

We classify real trivectors in dimension 9. The corresponding classification over the field C of complex numbers was obtained by Vinberg and Elashvili in 1978. One of the main tools used for their classification was the construction of the representation of SL(9,C) on the space of complex trivectors of C^9 as a theta-representation corresponding to a Z/3Z-grading of the simple complex Lie algebra of type E_8. This divides the trivectors into three groups: nilpotent, semisimple, and mixed trivectors. Our classification follows the same pattern. We use Galois cohomology, first and second, to obtain the classification over R., Comment: Shortened version (submitted to a journal) of arXiv:2106.00246 [math.RT]; 38 pages

Let G be a linear algebraic group over the field of real numbers R, and let Y be a right homogeneous space of G. We wish to find a real point of Y or to prove that Y has no real points. We describe a method to do that, implicitly using second nonabelian Galois cohomology. Our method is suitable for computer-assisted calculations., Comment: V1: 18 pages, V2: 22 pages. An appendix added

In this paper we classify real trivectors in dimension 9. The corresponding classification over the field C of complex numbers was done by Vinberg and Elashvili in 1978. One of the main tools used for their classification was the construction of the representation of SL(9,C) on the space of complex trivectors of C^9 as a theta-representation corresponding to a Z/3Z-grading of the simple complex Lie algebra of type E8. This divides the trivectors into three groups: nilpotent, semisimple, and mixed trivectors. Our classification follows the same pattern. We use Galois cohomology to obtain the classification over R. For the nilpotent orbits this is in principle rather straightforward and the main problem is to determine the first Galois cohomology sets of a long list of centralizers (we compute the centralizers using computer). For the semisimple and mixed orbits we develop new methods based on Galois cohomology, first and second. We also consider real theta-representations in general, and derive a number of results that are useful for the classification of their orbits., Comment: 137 pages

Let U be a unipotent group over the field of complex numbers C, acting on a complex algebraic variety X. Assume that there exists a surjective morphism of complex algebraic varieties f: X --> Y whose fibres are orbits of U. We show that if X and Y are smooth and all orbits of U in X have the same dimension, then the induced map on C-points X(C) --> Y(C) is a homotopy equivalence. Moreover, if U, X, Y, and f are defined over the field of real numbers R, then the induced map on R-points X(R) --> Y(R) is surjective and induces homotopy equivalences on connected components., Comment: 5 pages, comments are welcome!

By a quasi-connected reductive group (a term of Labesse) over an arbitrary field we mean an almost direct product of a connected semisimple group and a quasi-torus (a smooth group of multiplicative type). We show that a linear algebraic group is quasi-connected reductive if and only if it is isomorphic to a smooth normal subgroup of a connected reductive group. We compute the first Galois cohomology set H^1(R,G) of a quasi-connected reductive group G over the field R of real numbers in terms of a certain action of a subgroup of the Weyl group on the Galois cohomology of a fundamental quasi-torus of G., Comment: Version 1: 9 pages. Version 2: 12 pages. Version 2 supersedes arXiv:2108.05694. Starting this version, we consider quasi-connected reductive groups over a field of *arbitrary characteristic*. A new coauthor has been added

For a connected semisimple group G over the field of real numbers R, using a method of Onishchik and Vinberg, we compute the first Galois cohomology set H^1(R,G) in terms of Kac labelings of the affine Dynkin diagram of G., Comment: This preprint supersedes arXiv:1506.06252. V.1, 35 pages. V.2, 36 pages, final version to appear in Transformation Groups

Let $k_0$ be a field of characteristic $0$ with algebraic closure $k$. Let $G$ be a connected reductive $k$-group, and let $Y$ be a spherical variety over $k$ (a spherical homogeneous space or a spherical embedding). Let $G_0$ be a $k_0$-model ($k_0$-form) of $G$. We give necessary and sufficient conditions for the existence of a $G_0$-equivariant $k_0$-model of $Y$., Comment: This preprint supersedes arXiv:1804.08475. V1: 39 pages. V2: 44 pages; a section on spherical embeddings has been added, and therefore the title and abstract have been changed. V3: 60 pages; a section on horospherical homogeneous spaces has been added. V4: 64 pages; the final version to appear in IMRN

We study the equivariant real structures on complex horospherical varieties, generalizing classical results known for toric varieties and flag varieties. In particular, we obtain a necessary and sufficient condition for the existence of such real structures and determine the number of equivalence classes. We then apply our results to classify the equivariant real structures on smooth projective horospherical varieties of Picard rank 1., Comment: 33 pages, final version

Let k_0 be a field of characteristic 0, and let k be a fixed algebraic closure of k_0. Let G be an algebraic k-group, and let Y be a G-variety over k. Let G_0 be a k_0 -model (k_0 -form) of G. We ask whether Y admits a G_0 -equivariant k_0 -model Y_0 of Y. We assume that Y admits a G_q -equivariant k_0 -model Y_q, where G_q is an inner form of G_0. We give a Galois-cohomological criterion for the existence of a G_0 -equivariant k_0 -model Y_0 of Y. We apply this criterion to spherical homogeneous varieties Y=G/H., Comment: 14 pages, typos corrected

Let $G$ be a linear algebraic group over a field $k$ of characteristic 0. We show that any two connected semisimple $k$-subgroups of $G$ that are conjugate over an algebraic closure of $k$ are actually conjugate over a finite field extension of $k$ of degree bounded independently of the subgroups. Moreover, if $k$ is a real number field, we show that any two connected semisimple $k$-subgroups of $G$ that are conjugate over the field of real numbers $\mathbb{R}$ are actually conjugate over a finite real extension of $k$ of degree bounded independently of the subgroups., Comment: 13 pages; to appear in Proc. Amer. Math. Soc

We prove a Hasse principle for binary direct summands of the Chow motive of a smooth projective quadric Q over a number field F. Besides, we show that such summands are twists of Rost motives. In the case when F has at most one real embedding we describe a complete motivic decomposition of Q., Comment: 17 pages, accepted to International Mathematics Research Notices

Let $G$ be a connected semisimple group over an algebraically closed field $k$ of characteristic 0. Let $Y=G/H$ be a spherical homogeneous space of $G$, and let $Y'$ be a spherical embedding of $Y$. Let $k_0$ be a subfield of $k$. Let $G_0$ be a $k_0$ -model ($k_0$-form) of $G$. We show that if $G_0$ is an inner form of a split group and if the subgroup $H$ of $G$ is spherically closed, then $Y$ admits a $G_0$-equivariant $k_0$-model. If we replace the assumption that $H$ is spherically closed by the stronger assumption that $H$ coincides with its normalizer in $G$, then $Y$ and $Y'$ admit compatible $G_0$-equivariant $k_0$-models, and these models are unique., Comment: V2, 33 pages. A strong version of Losev's Uniqueness Theorem has been added. V3, 37 pages. Section 1 has been rewritten, an example due to Roman Avdeev has been added. V4, 40 pages. V5, 42 pages, final version, to appear in Transformation Groups

In this note, following Dedecker and Debremaeker, we extend the cohomology exact sequence for nonabelian $H^1$, using nonabelian $H^2$ with coefficients in crossed modules., Comment: 9 pages

Let G be a connected, compact, semisimple algebraic group over the field of real numbers R. Using Kac diagrams, we describe combinatorially the first Galois cohomology sets H^1(R,H) for all inner forms H of G. As examples, we compute explicitly H^1 for all real forms of the simply connected group of type E_7 (which has been known since 2013) and for all real forms of half-spin groups of type D_{2k} (which seems to be new)., Comment: 16 pages

Let X=G/H be the quotient of a connected reductive algebraic C-group G defined over the field of complex numbers C by a finite subgroup H. We describe the topological fundamental group of the homogeneous space X, which is nonabelian when H is nonabelian. Further, we construct an example of a homogeneous space X and an automorphism s of C such that the topological fundamental groups of X and of the conjugate variety sX are not isomorphic., Comment: 6 pages

Let G be a simply connected absolutely simple algebraic group defined over the field of real numbers R. Let H be a simply connected semisimple R-subgroup of G. We consider the homogeneous space X=G/H. We ask: How many connected components has X(R)? We give a method of answering this question. Our method is based on our solutions of generalized Reeder puzzles., Comment: 42 pages. The final version, to appear in J. Algebra

We describe functorially the first Galois cohomology set $H^1({\mathbb R},G)$ of a connected reductive algebraic group $G$ over the field $\mathbb R$ of real numbers in terms of a certain action of the Weyl group on the real points of order dividing 2 of the maximal torus containing a maximal compact torus. This result was announced with a sketch of proof in the author's 1988 note. Here we give a detailed proof., Comment: V.1, v.2, v.3: 6 pages. V.4, v.5: 11 pages, the final version to appear in Communicationa in Mathematics. In this final version, Theorem 9 (the main result) of versions 1-3 became Theorem 3.1

A linear algebraic group G over a field k is called a Cayley group if it admits a Cayley map, i.e. a G-equivariant birational isomorphism over k between the group variety G and its Lie algebra Lie(G). A prototypical example is the classical "Cayley transform" for the special orthogonal group SO(n) defined by Arthur Cayley in 1846. A linear algebraic k-group G is called stably Cayley if $G \times S$ is Cayley for some split k-torus S. We classify stably Cayley semisimple groups over an arbitrary field k of characteristic 0., Comment: 28 pages. The error noticed by the referee has been corrected

This is an English translation of the author's 1981 note in Russian, published in a Yaroslavl collection. We prove that if an Abelian variety over C has no nontrivial endomorphisms, then its Hodge group is Q-simple., Comment: 3 pages

Let k be a global field. Let G be a connected linear algebraic k-group, assumed reductive when k is a function field. It follows from a result of a preprint by Bary-Soroker, Fehm and Petersen that when H is a smooth connected k-subgroup of G, the quotient space G/H is of Hilbert type. We prove a similar result for certain non-connected k-subgroups H of G. In particular, we prove that if G is a simply connected k-group over a number field k, and H is an abelian k-subgroup of G, not necessarily connected, then G/H is of Hilbert type., Comment: 7 pages, final version, to appear in the International Journal of Number Theory

We define the algebraic fundamental group functor of a reductive group scheme over an arbitrary (non-empty) base scheme and prove that this functor is exact., Comment: 17 pages. This is an expanded version of Appendix B in arXiv:1112.6020v1[math.NT]. Final version, to appear in Cent. Eur. J. Math

Soit X un espace homog\`ene d'un groupe alg\'ebrique lin\'eaire connexe G sur C. Soit x un C-point de X. On d\'esigne par H le stabilisateur de x dans G. On montre qu'on peut d\'efinir alg\'ebriquement le groupe fondamental topologique \pi_1(X(C),x), si ce groupe fondamental topologique est ab\'elien. Si Pic(G)=0 et H est connexe ou ab\'elien, on calcule \pi_1(X(C),x) en termes des groupes de caract\`eres de G et H. En outre, si G et X sont d\'efinis sur un corps alg\'ebriquement clos de caract\'eristique p quelconque, on calcule la partie premi\`ere \`a p du groupe fondamental \'etale de X en termes des groupes de caract\`eres de G et H (si Pic(G)=0 et H est connexe). Let X be a homogeneous space of a connected linear algebraic group G defined over C. Let x be a C-point of X. We denote by H the stabilizer of x in G. We show that if the topological fundamental group \pi_1(X(C),x) is abelian, then it can be defined algebraically. If Pic(G)=0 and H is connected or abelian, we compute \pi_1(X(C),x) in terms of the character groups of G and H. Furthermore, when G and X are defined over an algebraically closed field of arbitrary characteristic p, we compute the prime-to-p \'etale fundamental group of X in terms of the character groups of G and H (if Pic(G)=0 and H is connected)., Comment: 23 pages, in French. The mistake, noticed by a referee, corrected

A linear algebraic group G is over a field K is called a Cayley K-group if it admits a Cayley map, i.e., a G-equivariant K-birational isomorphism between the group variety G and its Lie algebra. We classify real reductive algebraic groups of absolute rank 1 and 2 that are Cayley R-groups., Comment: 24 pages. Igor Dolgachev is the author of Appendix A. New Appendix B contributed by the anonymous referee has been added. Final version, to appear in J. Algebra

A linear algebraic group G is over a field K is called a Cayley group if it admits a Cayley map, i.e., a G-equivariant K-birational isomorphism between the group variety G and its Lie algebra. We prove that the special unitary group in 3 variables SU_3 is a Cayley group over R, thus extending a result of V.L. Popov, who proved in 1975 that the special linear group in 3 variables SL_3 over an algebraically closed field of characteristic 0 is Cayley. We also discuss the question whether the R-group SL_{3,R} is Cayley., Comment: This paper has been withdrawn by the author because it is superseded by arXiv:1212.1065

We give a new proof of the theorem stating that for any connected linear algebraic group G over an algebraically closed field k of characteristic 0 and for any closed connected subgroup H of G, the unramified Brauer group of G/H vanishes., Comment: 4 pages; shortened, misprints corrected

Let k be a field, G a smooth connected linear algebraic group and X a homogeneous space of G over k, such that the geometric stabilizers are extensions of a smooth group of multiplicative type by a smooth connected characterfree group. If k has characteristic zero and if X^c is a smooth compactification of X over k, we obtain a formula for the algebraic Brauer group of X^c. Several variants are obtained in positive characteristic p, including the finite field case and the global field case, where the formulae describe the prime-to-p part of the algebraic unramified Brauer group of X, without assuming the existence of a smooth compactification of X. Moreover, assuming that stabilizers are connected, then our formulae hold for the prime-to-p part of the whole unramified Brauer group., Comment: 42 pages

Let X be a homogeneous space, X = G/H, where G is a connected linear algebraic group over a number field k, and H is a k-subgroup of G (not necessarily connected). Let S be a finite set of places of k. We compute the Brauer-Manin obstruction to weak approximation for X in S in terms of Galois cohomology., Comment: 19 pages. Final version, to appear in Moscow Math. J

Let X be a homogeneous space of a quasi-trivial k-group G, with geometric stabilizer H, over a number field k. We prove that under certain conditions on the character group of H, certain algebraic Brauer-Manin obstructions to the Hasse principle and weak approximation vanish, because the abelian groups where they take values vanish. When H is connected or abelian, these algebraic Brauer-Manin obstructions to the Hasse principle and weak approximation are the only ones, so we prove the Hasse principle and weak approximation for X under certain conditions. As an application, we obtain new sufficient conditions for the Hasse principle and weak approximation for linear algebraic groups., Comment: V.5, 13 pages. Following a suggestion of the referee, the proofs of Lemmas 3.2 and 3.4 were omitted in the published version (see V.4). In this version 5, we give detailed proofs of those lemmas

Let k be a field of characteristic 0 and let X be a smooth geometrically integral k-variety. In our previous paper we defined the extended Picard complex UPic(X) as a certain complex of Galois modules in degrees 0 and 1. We computed the isomorphism class of UPic(G) in the derived category of Galois modules for a connected linear k-group G. In this paper we assume that X is a homogeneous space of a connected linear k-group G with geometric stabilizer H. We compute the isomorphism class of UPic(X) in the derived category of Galois modules in terms of the character groups of G and H. The proof is based on the notion of the extended equivariant Picard complex UPic_G(X) of a G-variety X., Comment: 32 pages. Final version, to appear in Transformation Groups