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On quadratic forms of height two and a theorem of Wadsworth
Authors:Detlev W. Hoffmann
Affiliation:Aindorferstr. 84, D-80689 Munich, Germany
Abstract:Let $varphi $ and $psi $ be anisotropic quadratic forms over a field $F$ of characteristic not $2$. Their function fields $F(% varphi )$ and $F(psi )$ are said to be equivalent (over $F$) if $% varphi otimes F(psi )$ and $psi otimes F(% varphi )$ are isotropic. We consider the case where $dim % varphi =2^n$ and $% varphi $ is divisible by an $(n-2)$-fold Pfister form. We determine those forms $psi $ for which $% varphi $ becomes isotropic over $F(psi )$ if $nleq 3$, and provide partial results for $ngeq 4$. These results imply that if $F(% varphi )$ and $F(psi )$ are equivalent and $dim % varphi =dim psi$, then $% varphi $ is similar to $psi $ over $F$. This together with already known results yields that if $% varphi $ is of height $2$ and degree $1$ or $2$, and if $dim % varphi =dim psi$, then $F(% varphi )$ and $F(psi )$ are equivalent iff $F(% varphi )$ and $F(psi )$ are isomorphic over $F$.

Keywords:Quadratic forms of height 2   function fields of quadratic forms   equivalence of function fields   isomorphism of function fields
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