patriot,
I assume from your post that you are looking for a method that does not involve using an analog multiplier in the feedback loop of an op-amp. One way to do this is to use a the log function. Use a log converter of your choosing to get the logarithm of the input. divide the log by 2. Then take the anti-log of the result. This is a lot messier than using an anlog multiplier.
Regards,
Kral