What does an $\oplus$-sign in the superscript mean? I've come across an expression $ M^{\oplus n_i}$ in an article and I have not seen this before? What does is mean? The whole expression looks like this: $$\large M=M_1^{\oplus n_1}\oplus M_2^{\oplus n_2} \oplus\ldots\oplus M_r^{\oplus n_r}$$

$M^{\bigoplus n}$ means a direct sum of $n$ copies of $M$. Another common notation is $M^{(n)}$. 
Likewise, $M^{\bigotimes n}$ means a tensor product of $n$ copies of $M$, etc.
