# Problem with ToBoundaryMesh

Bug introduced in 11.1.0 and fixed in 11.2.0

I have a problem that occurs in ToBoundaryMesh in Mathematica version 11.1, whereas in 11.0 it worked fine. I defined ImplicitRegion as shown below and it looks fine, but when I use this region in ToBoundaryMesh 2 corner points are lost (I checked it with select) and boundary is wrong. Consequently, also ToElementMesh gives wrong mesh. I am sorry about decimal numbers, example is taken from bigger model. Problem appears to be random, if I for example delete some decimal numbers in criterions for region result is correct.

<< NDSolveFEM
reg = ImplicitRegion[(-0.6666666666666666 + x)^2 + (-0.125 + y)^2 >=
0.0064 && (-0.6666666666666666 + x)^2 + (-0.375 + y)^2 >=
0.0064 && (-0.6666666666666666 + x)^2 + (-0.625 + y)^2 >=
0.0064 && (-0.6666666666666666 + x)^2 + (-0.875 + y)^2 >=
0.0064 && (-0.9333333333333333 + x)^2 + (-0.125 + y)^2 >=
0.0064 && (-0.9333333333333333 + x)^2 + (-0.375 + y)^2 >=
0.0064 && (-0.9333333333333333 + x)^2 + (-0.625 + y)^2 >=
0.0064 && (-0.9333333333333333 + x)^2 + (-0.875 + y)^2 >=
0.0064 && -y <= -0 && -1 + x <= 0 && -1 + y <=
0 && -x <= -0.5, {x, y}];
RegionPlot[reg]


boundary = ToBoundaryMesh[reg];
boundary["Wireframe"]
pp = boundary["Coordinates"];
Select[pp, #[[1]] == 1. && #[[2]] == 1. &]
Select[pp, #[[1]] == 1. && #[[2]] == 0. &]


 ToElementMesh[boundary]["Wireframe"]


Correct solution in Mathematica 11.0.

• That's a bug and fixed in the development version. – user21 Mar 31 '17 at 11:12
• Ok, so for now I will just use Mathematica 11. – NZupan Mar 31 '17 at 11:41

boundary =