Let $f_n(x,\ y)=\frac{n}{r\cos \pi r+n^2r^3}\ (r=\sqrt{x^2+y^2})$,

$I_n=\int\int_{r\leq 1} f_n(x,\ y)\ dxdy\ (n\geq 2).$

Find $\lim_{n\to\infty} I_n.$

[i]2009 Tokyo Institute of Technology, Master Course in Mathematics[/i]
1. **Convert the double integral to polar coordinates:**
   Given \( f_n(x, y) = \frac{n}{r \cos(\pi r) + n^2 r^3} \) where \( r = \sqrt{x^2 + y^2} \), we can convert the double integral \( I_n = \int \int_{r \leq 1} f_n(x, y) \, dx \, dy \) to polar coordinates. The Jacobian determinant for the transformation to polar coordinates is \( r \), so the integral becomes:
   \[
   I_n = \int_0^{2\pi} \int_0^1 \frac{n}{r \cos(\pi r) + n^2 r^3} r \, dr \, d\theta
   \]

2. **Simplify the integrand:**
   Simplify the integrand by multiplying \( r \) inside the fraction:
   \[
   I_n = \int_0^{2\pi} \int_0^1 \frac{n r}{r \cos(\pi r) + n^2 r^3} \, dr \, d\theta = \int_0^{2\pi} \int_0^1 \frac{n r}{r (\cos(\pi r) + n^2 r^2)} \, dr \, d\theta = \int_0^{2\pi} \int_0^1 \frac{n}{\cos(\pi r) + n^2 r^2} \, dr \, d\theta
   \]

3. **Separate the angular part:**
   The integral with respect to \(\theta\) is straightforward:
   \[
   I_n = \int_0^{2\pi} d\theta \int_0^1 \frac{n}{\cos(\pi r) + n^2 r^2} \, dr = 2\pi \int_0^1 \frac{n}{\cos(\pi r) + n^2 r^2} \, dr
   \]

4. **Analyze the integrand as \( n \to \infty \):**
   For large \( n \), the term \( n^2 r^2 \) will dominate over \( \cos(\pi r) \) for \( r \in [0, 1] \). Therefore, we can approximate the integrand:
   \[
   \frac{n}{\cos(\pi r) + n^2 r^2} \approx \frac{n}{n^2 r^2} = \frac{1}{n r^2}
   \]

5. **Evaluate the integral:**
   Substitute the approximation into the integral:
   \[
   2\pi \int_0^1 \frac{1}{n r^2} \, dr
   \]
   This integral is improper at \( r = 0 \), so we need to handle it carefully:
   \[
   2\pi \int_0^1 \frac{1}{n r^2} \, dr = \frac{2\pi}{n} \int_0^1 \frac{1}{r^2} \, dr
   \]
   The integral \( \int_0^1 \frac{1}{r^2} \, dr \) diverges, but we need to consider the behavior as \( n \to \infty \). To handle this, we can use a more precise bound.

6. **Bounding the integral:**
   Notice that for \( r \in [0, 1] \), \( \cos(\pi r) \) is bounded between \(-1\) and \(1\). Therefore:
   \[
   \cos(\pi r) + n^2 r^2 \geq n^2 r^2 - 1
   \]
   Hence:
   \[
   \frac{n}{\cos(\pi r) + n^2 r^2} \leq \frac{n}{n^2 r^2 - 1}
   \]
   For large \( n \), \( n^2 r^2 - 1 \approx n^2 r^2 \), so:
   \[
   \frac{n}{n^2 r^2 - 1} \approx \frac{1}{n r^2}
   \]

7. **Conclusion:**
   As \( n \to \infty \), the integral \( \int_0^1 \frac{1}{n r^2} \, dr \) tends to zero because the factor \( \frac{1}{n} \) dominates. Therefore:
   \[
   \lim_{n \to \infty} I_n = \lim_{n \to \infty} 2\pi \int_0^1 \frac{1}{n r^2} \, dr = 0
   \]

The final answer is \(\boxed{0}\)