The code in this codex page for "pre_option_posts_per_page" filter is not correct ...
http://codex.wordpress.org/Plugin_API/Filter_Reference/pre_option_(option_name)
Because I copied the code verbatim in attempt to alter the amount of displayed posts per page for a specific category. And the code did not take effect (although there was not error in output).
Can someone who knows enough about wordpress core tell me the correct code for doing this?