# Problem with a double integral

Why is the following integral not evaluated by Mathematica (ver. 11.3, Windows 10)

Integrate[1, {x, 2 - Sqrt[2], 1}, {y,
1/2 - 1/2 Sqrt[(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)],
1/2 + 1/2 Sqrt[(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)]}]


Edit: This is actually a double integral $$\int_{2-\sqrt{2}}^{1} dx \int_{ \frac{1}{2} - \frac{1}{2} \sqrt{(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)}}^{ \frac{1}{2} + \frac{1}{2} \sqrt{(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)}} dy$$.

• What integral do you want to solve? Could you write it out or post a picture?
– Tomi
Commented Aug 28, 2021 at 19:51
• This integral is difficult to solve analytically, but simple numerically. Therefore, use NIntegrate instead of Integrate Commented Aug 28, 2021 at 20:12

I am confused as to what integral you want to be solved.

You need to write the integral function in a form like

(*integrand*)
eq = 1;
(*limits*)
limits = {x, 2 - Sqrt[2], 1};

result = Integrate[eq, limits]


So, I don't understand your question - are you trying to integrate it twice?

eq = 1;
limits = {y,
1/2 - 1/2 Sqrt[(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)],
1/2 + 1/2 Sqrt[(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)]};
result1 = Integrate[eq, limits]
limits2 = {x, 2 - Sqrt[2], 1};
NIntegrate[result1, limits2]

• I have edited my question. What I wrote is actually a double integral. Commented Aug 28, 2021 at 20:01
– Tomi
Commented Aug 28, 2021 at 20:05

As an alternative to @Tomi's solution, define a two-dimensional region:

R = ImplicitRegion[x*(x-2)*(x*y*(x-2)*(y-1)+1)+1 <= 0 &&
2 - Sqrt[2] <= x <= 1,
{x, y}];


Numerical integration over regions is straightforward:

NIntegrate[1, Element[{x, y}, R]]
(*    0.339303    *)


You can of course also use your original parametrization of the region:

R0 = ImplicitRegion[2 - Sqrt[2] <= x <= 1 &&
1/2 - 1/2 Sqrt[(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)] <= y <=
1/2 + 1/2 Sqrt[(-4 + 8 x - 4 x^3 + x^4)/((-2 + x)^2 x^2)],
{x, y}];
NIntegrate[1, Element[{x, y}, R0]]
(*    0.339303    *)