behavior of places of a function field under automorphism - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-24T01:03:48Z http://mathoverflow.net/feeds/question/32623 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/32623/behavior-of-places-of-a-function-field-under-automorphism behavior of places of a function field under automorphism y_kaplan 2010-07-20T12:10:54Z 2010-07-20T13:37:16Z <p>if $P_{1}$ and $P_{2}$ are distinct places of equal degree of the function field F/K, and $\sigma$ is a K-field automorphism, such that $\sigma(P_{1})=P_{2}$. then, does $\deg (P_{1}\cap K(x))=\deg (P_{2}\cap K(x))$, where K(x) is the rational function field? in particular, is this true over the hermitian function field? </p> http://mathoverflow.net/questions/32623/behavior-of-places-of-a-function-field-under-automorphism/32636#32636 Answer by Hagen for behavior of places of a function field under automorphism Hagen 2010-07-20T13:37:16Z 2010-07-20T13:37:16Z <p>No, not in general, that is not without particular requirements for $x$:</p> <p>take $F=\mathbb{R}(y)$, the rational function field in one variable over the reals. Then the equation $\sigma (y)=y+1$ determines an automorphism of $F/\mathbb{R}$.</p> <p>Let $P_1$ be the place associated to the polynomial $y^2+1$; then $\deg (P_1)=2$.</p> <p>Let $P_2 := \sigma (P_1)$; then $P_2$ is associated to the polynomial $y^2+2y+2$ and (automatically) $\deg (P_2)=2$.</p> <p>Let $x := y^2+1$; then $[F:\mathbb{R}(x)]=2$ and $P_1|_{\mathbb{R}(x)}$ has degree $1$.</p> <p>On the other hand $yP_2$ either equals $i-1$ or $-i-1$. In both cases $xP_2$ is non-real and thus $\deg (P_2)=2$.</p> <p>H</p>