I have a following implicit region: Cosalpha[x_, y_, cosz_] = cosz*Sin[x]*Sin[y] + Cos[x]*Cos[y]; Cosalphamin[E1_, m1_, m2_] = 1/(2*m2)*Sqrt[4 E1^2 m2^2 - m2^4 - 2 m2^2 m1^2 - m1^4]/ Sqrt[E1^2 - m1^2] E1crit[m1_, m2_] = (m1^2 + m2^2)/(2*m2); coszmin[m1_, E1_, m2_, x_, y_] = If[E1 > E1crit[m1, m2], cosz /. Solve[Cosalpha[x, y, cosz] - cosalphamin == 0, cosz][[1]] /. cosalphamin -> Cosalphamin[E1, m1, m2], -1] Regionxy[m1_, E1_, m2_] := ImplicitRegion[ coszmin[m1, E1, m2, x, y] <= 1, {{x, 0, Pi}, {y, 0, Pi}}] When I try to visualize the region using RegionPlot for E1 > E1crit, Mathematica stucks: RegionPlot[Regionxy[2, 5*E1crit[2, 6.3], 6.3]] I don't understand why it fails. Is there some qualitative reason, or it is simply numerical? Anyway, is there some way to make the evaluation faster? Login To add answer/comment