I cannot understand why I cannot reproduce an example given in the NDSolve command help.
Namely I would like to plot the steepest ascent path on the surface $z = x^2-y^2$, starting from the point $(0,-1)$. I try to solve the system of differential equations for the gradient flow
s = NDSolve[{x'[t] == 2*x[t], y'[t] == -2*y[t], x[0] == 0,
y[0] == -1}, {x, y}, {t, 20}]
but when I issue the command, as in the command help
ParametricPlot[Evaluate[{x[t], y[t]} /. s], {t, 0, 20}]
the axes appear, but not the expected plot, a segment from $(-1,0)$ to $(0,0)$, thanks for any hint