Notice that in [9] there are five odd numbers and four even.
The even 'separate' the odds: "_2_4_6_8_". Fill in the blanks with the odds.
There (4!) ways to arrange the evens and then (5!) ways to arrange the odds.
SO?
How many permutations of [9] have no adjacent odd digits?
First look at the same problem for [5] we have:
12345 12543 14325 14523
32145 32541 34125 34521
52341 52143 54321 54123
This suggests that for each odd digit we have a permutation of the rest of the even digits times the permutation of the rest of the odd digits (I think) giving
= 12
Then the answer to the original question would be
= 5184
Does this solution hold any merit or am I totally confused as usual?
Thanks