Major Description
Take a look at the typical math functions that are implemented in relational database systems (http://www.postgresql.org/docs/current/static/functions-math.html) and implement the same for Phoenix in Java following this guide: http://phoenix-hbase.blogspot.com/2013/04/how-to-add-your-own-built-in-function.html
For this specific task, it is meant to implement missing funstion POWER for Phoenix in Java.