6. Given 2015 positive integers $a_{1}, a_{2}, \cdots, a_{2015}$ satisfying
$$
\begin{array}{l}
a_{1}=1, a_{2}=8, \\
a_{n+1}=3 a_{n}-2 a_{n-1}(n \geqslant 2, \text { and } n \in \mathbf{N}) .
\end{array}
$$

Then the sum of all positive divisors of $a_{2015}-a_{2014}$ is $\qquad$