# ParametricPlot3D of Boy's surface

I'm trying to visualize Boy's surface using Bryant's parametrization, as per the MathWorld article. However, I'm not sure I understand the parametrization, and I don't know how to implement it when the parameter is complex.

z := u + vI
g1 := -(3/2) Im[(z (1 - z^4))/(z^6 + z^3 √5 - 1)]
g2 := -(3/2) Re[(z (1 + z^4))/(z^6 + z^3 √5 - 1)]
g3 := Im[(1 + z^6)/(z^6 + z^3 √5 - 1)] - 1/2
g = g1^2 + g2^2 + g3^2

ParametricPlot3D[{g1/g, g2/g, g3/g}, {u, 0, 0.5}, {v, 0, 0.5}]


Now, I understand that $z=u+v\mathrm{i}$ is the parameter, and that it must range over the closed unit disc, so $u^2+v^2\leq1$. But I don't know how do do this! (I let $u,v$ range between 0 and 0.5, for good measure, but the graphic appears empty.)

In the end, I want to end up with exactly this image, for which I've been unable to find the original code.

Edit: The 3 solutions provided here worked like a charm (with the caveat that the one using polar coordinates was weirdly sharped, and I couldn't fix that, but I'm a Mathematica newb), and I found another method:

ParametricPlot3D[Boys[u, v], {u, v} ∈ Disk[{0, 0}, 1]]


I don't know why, but using RegionFunction the surface rendered ~30% slower, and, using Piecewise, about %5 slower. The only issue is that the mesh is now weirdly triangulated.

Also, I compiled the actual parametric formula and performance increased threefold:

Boys = Compile[{u,
v}, {(-(3/2)
Im[((u + v I) (1 - (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])/((-(3/2)
Im[((u + v I) (1 - (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])^2 + (-(3/2)
Re[((u + v I) (1 + (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])^2 + (Im[(
1 + (u + v I)^6)/((u + v I)^6 + (u + v I)^3 Sqrt - 1.)] -
1/2)^2), (-(3/2)
Re[((u + v I) (1 + (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])/((-(3/2)
Im[((u + v I) (1 - (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])^2 + (-(3/2)
Re[((u + v I) (1 + (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])^2 + (Im[(
1 + (u + v I)^6)/((u + v I)^6 + (u + v I)^3 Sqrt - 1.)] -
1/2)^2), (Im[(
1 + (u + v I)^6)/((u + v I)^6 + (u + v I)^3 Sqrt - 1.)] - 1/
2)/((-(3/2)
Im[((u + v I) (1 - (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])^2 + (-(3/2)
Re[((u + v I) (1 + (u + v I)^4))/((u +
v I)^6 + (u + v I)^3 Sqrt - 1.)])^2 + (Im[(
1 + (u + v I)^6)/((u + v I)^6 + (u + v I)^3 Sqrt - 1.)] -
1/2)^2)}, Parallelization -> True];


I also tried compiling each of $g1,g2,g3,g$ separately, but performance was a bit worse.

• Hello! It's g = g1^2+g2^2+g3^2, right? – Ivan May 1 '15 at 20:25
• Yes! Will correct! – étale-cohomology May 1 '15 at 20:27
• Your first problem is that vI is a different variable name; you need v I with a space in between. – user484 May 1 '15 at 20:41
• "that it must range over the closed unit disc" - that should have tipped you off to use polar coordinates. As I don't have Mathematica on hand, try this: ParametricPlot3D[With[{z = r Exp[I θ]}, (#/(#.#)) &[Re[{I z (1 - z^4), z (z^4 + 1), -(2 I/3) (z^6 + 1)}/(z^6 + Sqrt z^3 - 1) + {0, 0, 1/2}]]], {r, 0, 9/10}, {θ, -π, π}, PlotPoints -> {20, 30}] – J. M.'s technical difficulties May 2 '15 at 1:22
• Yes, I totally missed that!! Thanks! – étale-cohomology May 2 '15 at 17:21

z := u + v I
g1 := -(3/2) Im[(z (1 - z^4))/(z^6 + z^3 Sqrt - 1)]
g2 := -(3/2) Re[(z (1 + z^4))/(z^6 + z^3 Sqrt - 1)]
g3 := Im[(1 + z^6)/(z^6 + z^3 Sqrt - 1)] - 1/2
g = g1^2 + g2^2 + g3^2;


You could use Piecewise here.

When Abs[z] > 1, the function is undefined and plots nothing.

ParametricPlot3D[Piecewise[{{{g1/g, g2/g, g3/g}, Abs[z] <= 1}}, Undefined],
{u, -1, 1}, {v, -1, 1}] I'm not an expert on Graphics, but I hope that helps.

• You could also add the option RegionFunction -> Function[{x, y, z, u, v}, u^2 + v^2 <= 1] instead of changing the function. – user484 May 1 '15 at 20:42
• Ah, yes, both solutions worked!! Thanks so much! On a totally unrelated note, does anyone happen to know of this parametrization has 3-fold symmetry when viewed from the 'top'? – étale-cohomology May 1 '15 at 21:06
• @Diego, yes, my current Gravatar does display that feature you speak of... – J. M.'s technical difficulties May 2 '15 at 0:33

Altho traditionally one should use polar coordinates as I suggested in a comment above, I found it amusing that ParametricPlot3D[] can actually support a region specification for specifying the domain of the parameters. Thus here is a way to plot the Bryant-Kusner parametrization of the Boy surface:

ParametricPlot3D[With[{z = u + I v}, (#/(#.#)) &[Re[{0, 0, 1/2} +
{I z (1 - z^4), z (z^4 + 1), -(2 I/3) (z^6 + 1)}/
(z^6 + Sqrt z^3 - 1)]]], {u, v} ∈ Disk[],
Axes -> None, Boxed -> False, Mesh -> True,
PlotPoints -> {20, 30}, PlotStyle -> Opacity[2/3]] where I made the surface translucent to display the inner structure. Mathematically, the construction is an inversion of a certain minimal surface that is topologically equivalent to the projective plane, thrice-punctured. See the linked paper for more background.