Definition:Multiindex/Modulus

Definition
Let $k = \left \langle {k_j}\right \rangle_{j \mathop \in J}$ be a multiindex.

The modulus of such a multiindex $k$ is defined by:
 * $\displaystyle \left|{k}\right| = \sum_{j \mathop \in J} k_j$

Note that, since by definition all but finitely many of the $k_j$ are zero, this sum converges.