It is a requirement for members who post questions in this subforum to:
1. Specifically say at the start of your question that it is a challenge question.
2. You must pm the solution to all Global Moderators (the ones whose names are green) unless you are an Expert, Helper, Ambassador, Administrator or Moderator. A list of all forum leaders (including Global Moderators) can be found here: http://www.mathhelpforum.com/math-help/showgroups.php.
If you do not do these two things then I'm sorry but your question will be deleted and you will receive an Infraction.
I apologise for for this draconian criteria. However, members have been consistently abusing this subforum by posting questions they cannot do. This is unacceptable.
Further information can be read in the other sticky.