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In the mathematical field of differential geometry a Liouville surface^[1] (named after Joseph Liouville) is a type of surface which in local coordinates may be written as a graph in R³

${\displaystyle z=f(x,y)}$

such that the first fundamental form is of the form

${\displaystyle ds^{2}={\big (}f_{1}(x)+f_{2}(y){\big )}\left(dx^{2}+dy^{2}\right).}$

Sometimes a metric of this form is called a Liouville metric. Every surface of revolution is a Liouville surface.

Darboux^[2] gives a general treatment of such surfaces considering a two-dimensional space ${\displaystyle (u,v)}$ with metric

${\displaystyle ds^{2}=(U-V)(U_{1}^{2}\,du^{2}+V_{1}^{2}\,dv^{2}),}$

where ${\displaystyle U}$ and ${\displaystyle U_{1}}$ are functions of ${\displaystyle u}$ and ${\displaystyle V}$ and ${\displaystyle V_{1}}$ are functions of ${\displaystyle v}$. A geodesic line on such a surface is given by

${\displaystyle {\frac {U_{1}\,du}{\sqrt {U-\alpha }}}-{\frac {V_{1}\,dv}{\sqrt {\alpha -V}}}=0}$

and the distance along the geodesic is given by

${\displaystyle ds={\frac {UU_{1}\,du}{\sqrt {U-\alpha }}}-{\frac {VV_{1}\,dv}{\sqrt {\alpha -V}}}.}$

Here ${\displaystyle \alpha }$ is a constant related to the direction of the geodesic by

${\displaystyle \alpha =U\sin ^{2}\omega +V\cos ^{2}\omega ,}$

where ${\displaystyle \omega }$ is the angle of the geodesic measured from a line of constant ${\displaystyle v}$. In this way, the solution of geodesics on Liouville surfaces is reduced to quadrature. This was first demonstrated by Jacobi for the case of geodesics on a triaxial ellipsoid,^[3] a special case of a Liouville surface.