What's their argument?
This question has been asked before.
http://www.reddit.com/r/AskHistorians/comments/1n3vwo/do_holocaust_deniers_have_any_valid_points/
http://www.reddit.com/r/AskHistorians/comments/1h0ekm/how_does_one_become_a_holocaust_denier_and_what/
http://www.reddit.com/r/AskHistorians/comments/1kq1wr/where_do_the_holocaust_deniers_even_get_the_idea/