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I want to evaluate an integral that involves two disjoint unit disks $D_1$ and $D_2$. $D_1$ is centered at $(-2,0)$ and $D_2$ is centered at $(0,2)$. The integral I want to compute is

$$I = \int_{D_1} \int_{D_2} \log|x-y| dy dx.$$

I looked at the in-built Python integration methods and also the quadpy library but although they have lots of options for integration over a single disk, I couldn't find anything that can help me with integrating over two disjoint disks.

Is it possible to evaluate this integral in Mathematica? I don't need an optimum method, I just need to obtain the value of this integral.

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    $\begingroup$ The exact value of the integral under consideration is $\pi^2\log 4$. The function $\log |x-y|$ is harmonic and the mean value property can be twice applied (see en.wikipedia.org/wiki/Harmonic_function). $\endgroup$
    – user64494
    Feb 2, 2019 at 21:18
  • $\begingroup$ Mathematica calculates the integral under consideration, producing $\frac{1}{4} \pi ^2 (-1+2 i \pi +\log (16)) $. A bug was submitted by me. $\endgroup$
    – user64494
    Feb 3, 2019 at 18:51

1 Answer 1

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NIntegrate[
 Log[Norm[{x1, x2} - {y1, y2}]], 
 {x1, x2} ∈ Disk[{-2, 0}, 1], 
 {y1, y2} ∈ Disk[{2, 0}, 1]
 ]

13.6822

(the actual result being 13.682176919165677),

In order to enter ∈, just type Esc e l Esc.

In order to increase the precision, use the option PrecisionGoal.

Edit

Another possibility that relieves Mathematica from the need to discretize the disks and that allows her to use higher-order quadrature formulas is to employ polar coordinates on each of the disks:

NIntegrate[
 Log[Norm[{r1 Cos[θ1] - 2,r1 Sin[θ1]} - {r2 Cos[θ2] + 2,r2 Sin[θ2]}]] r1 r2,
 {r1, 0, 1}, {θ1, -Pi, Pi},
 {r2, 0, 1}, {θ2, -Pi, Pi},
 Method -> "LocalAdaptive"
 ]
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  • $\begingroup$ Whoah, that's very nice thanks! I actually just managed to get it going in with some manual change of variables in Python with nquad and I got the result $13.682176927714263$ $\endgroup$
    – ManUtdBloke
    Feb 2, 2019 at 15:55
  • $\begingroup$ You're welcome. $\endgroup$
    – Henrik Schumacher
    Feb 2, 2019 at 15:57
  • $\begingroup$ Great, I just needed a decent approximation, even the first 4 decimals places would suffice but there's no harm being even more accurate! $\endgroup$
    – ManUtdBloke
    Feb 2, 2019 at 15:57
  • $\begingroup$ You can also lower the PrecisionGoal for more speed... $\endgroup$
    – Henrik Schumacher
    Feb 2, 2019 at 15:58
  • $\begingroup$ Use of Norm is not a good idea since this equals Sqrt[Abs[x1 - x2]^2 + Abs[y1 - y2]^2]. $\endgroup$
    – user64494
    Feb 2, 2019 at 20:09

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