# Notation question - what does ! mean?

1. Oct 8, 2009

Notation question - what does "!" mean?

1. The problem statement, all variables and given/known data
Hi, this is part of a question, in the context of rotation of the Lagrangian:

x_i -> $$O$$_ij.x_j, summation over j !

My question is: what does the " !" after the j mean?

Thanks

2. Relevant equations

3. The attempt at a solution

2. Oct 8, 2009