Notation question - what does mean?

[Pyroadept]

Notation question - what does "!" mean?

Homework Statement

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

Homework Equations

The Attempt at a Solution

 

[tiny-tim]

Hi Pyroadept!

(try using the X₂ tag just above the Reply box )

x_i -> O_ijx_j is written in the "Einstein summation convention", where any repeated index is summed over.

I don't think the ! means anything, except happiness !