# Draw the level curves defined by an implicit integral equation?

GROUPS:
 Hello, I have a problem in the process of drawing characteristics of a PDE in the (tau,chi) plane. My characteristics are defined by the implicit equation [1] where I vary the parameter Gamma and I take the + equation. Note that X, v and cs are functions of (tau,chi) defined as in the code bellow.My attempt to draw the level curves defined by this equation is: L[X_] := X + X^2/2 ; cs[X_] := sqrt[(L'[X])/(2*X*L''[X] + L'[X])]; g[X_] := Integrate[1/(X*cs[X]) , X]; (* Primmitive of dX/X*cs *) v[\[Tau]_ , \[Chi]_] := - \[Chi]/\[Tau]; f1[\[Tau]_, \[Chi]_] := g[(\[Tau]^2 - \[Chi]^2)/2] + Log[(1 + v[\[Tau], \[Chi]])/(1 - v[\[Tau], \[Chi]])]; ContourPlot[ f1[\[Tau], \[Chi]] , {\[Tau], -1.5, 1.5}, {\[Chi], -1.5, 1.5}, PlotLegends -> Automatic] My problem is that the primitive g[X] corresponding to the integral in [1] is undefined.Do you have an idea of a solution?Many thanks.
 Gianluca Gorni 3 Votes Capitalize Sqrt and define g with immediate Set: cs[X_] := Sqrt[(L'[X])/(2*X*L''[X] + L'[X])]; Clear[X]; g[X_] = Integrate[1/(X*cs[X]), X];