content_image/295518.jpg content_image/295519.jpg content_image/295520.jpg content_image/295521.jpg
Then 

We immediately arrive at our final result: 

## Proca 

<img src='content_image/295518.jpg'>

<img src='content_image/295519.jpg'>

When we include an external current in the Proca Lagrangian we have 

The EOM are then 

<img src='content_image/295520.jpg'>

<img src='content_image/295521.jpg'>

