Is there a common symbol for concatenating two (finite) sequences? Say we have two finite sequences $X = (x_0,...,x_n)$ and $Y = (y_0,...,y_n)$.
Is there a more or less common notation for the concatenation of these sequences, like $\sum (X,Y) = (x_0,...,x_n,y_0,...,y_n)$?
The comments suggest the following notations for the concatenation of $X$ and $Y$:


$X^\frown Y$ (given by X^\frown Y);
$XY$ (given by XY);
$X \cdot Y$ (given by X \cdot Y);
$X \mathbin\Vert Y$ (given by X \mathbin\Vert Y);


of which the first seems not to be in use for other concepts, making it especially suitable.
Starting with your defined sequences $X = (x_0, \ldots, x_n)$ and $Y=(y_0,\ldots,y_n)$, you can use the commonly accepted tuple/ordered pair notation:
\begin{align}
   (X,Y) &amp;= \left(
    \left(x_0,\ldots,x_n\right),
    \left(y_0,\ldots,y_n\right)
   \right)
   \\
   &amp;= \left(x_0,\ldots,x_n,y_0,\ldots,y_n\right)
  \end{align}