# Finding the number of ways

• Oct 12th 2011, 03:09 AM
pranay
Finding the number of ways
Hi, if we have an initial arrangement say 12345 where all elements are unique, then in how many ways can one re-arrange them so that in the final configurations none of the elements which were adjacent to each other in the given arrangement are adjacent now in the final configurations.?
Does this involve some series?
E.g for 5 element say 12345, the number of such arrangements is 14.
Thanks.
• Oct 12th 2011, 05:26 AM
Plato
Re: Finding the number of ways
Quote:

Originally Posted by pranay
Hi, if we have an initial arrangement say 12345 where all elements are unique, then in how many ways can one re-arrange them so that in the final configurations none of the elements which were adjacent to each other in the given arrangement are adjacent now in the final configurations.? Does this involve some series?
E.g for 5 element say 12345, the number of such arrangements is 14.

How did you get that 14?

Note that for $123$ there are no cases.

How many are there for $1234$?
Is this one of those, $3142~?$
Given that string, there are three places to put a 5: $53142,~35142,~31425$.

So if we know how may valid cases there are for $1234$ we can build valid cases for $12345$.

I will let you work on that.
• Oct 12th 2011, 07:16 AM
pranay
Re: Finding the number of ways
if the initial configuration is ABCDE , then the 14 required configurations are:
ACEBD