The domain of the function $f(x) = \text{arcsin}(\log_{m}(nx))$ is a closed interval of length $\frac{1}{2013}$, where $m$ and $n$ are positive integers and $m > 1$. Find the remainder when the smallest possible sum $m+n$ is divided by $1000$.
1. The function given is \( f(x) = \arcsin(\log_m(nx)) \). The domain of the arcsin function is \([-1, 1]\). Therefore, we need to find the values of \(x\) such that \(-1 \leq \log_m(nx) \leq 1\).

2. To solve for \(x\), we start by solving the inequalities:
   \[
   -1 \leq \log_m(nx) \leq 1
   \]

3. Converting the logarithmic inequalities to exponential form, we get:
   \[
   m^{-1} \leq nx \leq m^1
   \]

4. Simplifying these inequalities, we have:
   \[
   \frac{1}{m} \leq nx \leq m
   \]

5. Dividing through by \(n\), we obtain:
   \[
   \frac{1}{mn} \leq x \leq \frac{m}{n}
   \]

6. The length of the interval \(\left[\frac{1}{mn}, \frac{m}{n}\right]\) is given by:
   \[
   \frac{m}{n} - \frac{1}{mn}
   \]

7. According to the problem, this length is \(\frac{1}{2013}\). Therefore, we set up the equation:
   \[
   \frac{m}{n} - \frac{1}{mn} = \frac{1}{2013}
   \]

8. To solve for \(n\), we first find a common denominator:
   \[
   \frac{m^2 - 1}{mn} = \frac{1}{2013}
   \]

9. Multiplying both sides by \(2013mn\), we get:
   \[
   2013(m^2 - 1) = n
   \]

10. Rearranging, we find:
    \[
    n = \frac{2013(m^2 - 1)}{m}
    \]

11. To minimize \(n\), we need to minimize \(m\). Since \(m\) must be a positive integer greater than 1, we start with the smallest possible value for \(m\), which is 2.

12. Substituting \(m = 3\) (since \(m = 2\) does not yield an integer \(n\)):
    \[
    n = \frac{2013(3^2 - 1)}{3} = \frac{2013 \cdot 8}{3} = 5368
    \]

13. The sum \(m + n\) is:
    \[
    m + n = 3 + 5368 = 5371
    \]

14. Finally, we find the remainder when 5371 is divided by 1000:
    \[
    5371 \mod 1000 = 371
    \]

The final answer is \(\boxed{371}\).