I will explain why they don't exsist simply by reminding everyone what a time signature is........the top number tells us how many beats there are in a measure...the bottom number tells us what type of note gets a count of "one". as an example, alternating bars of 11/8 & 13/8 is possible because the bottom number tells us that an eighth note counts as one beat......alternating bars of 11/9 & 13/9 are not possible because there is no ninth note.......hope this helps

steve