Lluís, You could use http://drupal.org/project/computed_field to store the computed data on the node itself, then filter on that data in the view.
Otherwise, if you do not want to store redundant data, and want to do the math at the view runtime, you could look into a custom module solution that uses hook_views_tables() and custom written handlers.
Seth
Lluís wrote:
I have a view with two fields (A and B), both of them numeric. I want to make a filter to show nodes when A + B = 0
How can I make this? I suppose I should add A and B as hidden fields and then there should be a calculated field anywhere allowing to add(), concatenate(), .... But I have been unable to find this module.
Thanks