An anisotropic algebraic group over a field $k$ is a linear algebraic group $G$ defined over $k$ and of $k$-rank zero, i.e. not containing non-trivial $k$-split tori . Classical examples of anisotropic groups include the orthogonal groups of quadratic forms that do not vanish over $k$; and algebraic groups of elements of reduced norm one in division algebras over $k$. If $G$ is semi-simple, and if the characteristic of $k$ is zero, then $G$ is anisotropic over $k$ if and only if $G_k$ contains no non-trivial unipotent elements. (For the field of real numbers or the field of $p$-adic numbers this is equivalent to saying that $G_k$ is compact.) The classification of arbitrary semi-simple groups over the field $k$ reduces essentially to the classification of anisotropic groups over $k$ .
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Anisotropic group. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Anisotropic_group&oldid=19932