Find the integer represented by $\left[ \sum_{n=1}^{10^9} n^{-2/3} \right] $. Here $[x]$ denotes the greatest integer less than or equal to $x.$
1. We need to find the integer part of the sum $\left[ \sum_{n=1}^{10^9} n^{-2/3} \right]$. To do this, we will approximate the sum using integrals.

2. Consider the integral $\int_1^{10^9} x^{-\frac{2}{3}} \, dx$. This integral will help us approximate the sum $\sum_{n=1}^{10^9} n^{-2/3}$.

3. Compute the integral:
   \[
   \int_1^{10^9} x^{-\frac{2}{3}} \, dx = \left[ \frac{x^{1 - \frac{2}{3}}}{1 - \frac{2}{3}} \right]_1^{10^9} = \left[ \frac{x^{\frac{1}{3}}}{\frac{1}{3}} \right]_1^{10^9} = 3 \left[ x^{\frac{1}{3}} \right]_1^{10^9}
   \]
   \[
   = 3 \left( (10^9)^{\frac{1}{3}} - 1^{\frac{1}{3}} \right) = 3 \left( 10^3 - 1 \right) = 3 \times 999 = 2997
   \]

4. Next, we use the integral bounds to approximate the sum:
   \[
   \int_1^{10^9} x^{-\frac{2}{3}} \, dx > \sum_{n=2}^{10^9} n^{-\frac{2}{3}} > \int_2^{10^9+1} x^{-\frac{2}{3}} \, dx
   \]

5. Compute the lower bound integral:
   \[
   \int_2^{10^9+1} x^{-\frac{2}{3}} \, dx = 3 \left[ x^{\frac{1}{3}} \right]_2^{10^9+1} = 3 \left( (10^9+1)^{\frac{1}{3}} - 2^{\frac{1}{3}} \right)
   \]
   \[
   \approx 3 \left( 1000 - 1.26 \right) = 3 \times 998.74 = 2996.22
   \]

6. Therefore, we have:
   \[
   2997 > \sum_{n=2}^{10^9} n^{-\frac{2}{3}} > 2996.22
   \]

7. Adding the first term $1^{-2/3} = 1$ to the sum:
   \[
   2997 + 1 > \sum_{n=1}^{10^9} n^{-\frac{2}{3}} > 2996.22 + 1
   \]
   \[
   2998 > \sum_{n=1}^{10^9} n^{-\frac{2}{3}} > 2997.22
   \]

8. Hence, the greatest integer less than or equal to $\sum_{n=1}^{10^9} n^{-2/3}$ is:
   \[
   \left\lfloor \sum_{n=1}^{10^9} n^{-2/3} \right\rfloor = 2997
   \]

The final answer is $\boxed{2997}$