Function which gives back the nearest neighbours of a requested value

Hi!

I'm looking for an existing function which allows me to search the nearest
neighbours of the requested value.

A simple example: if I search the value "2", and my table doesn't contain
it, but only contains "0" and "10", as "nearest neighbours", then it will
give me back these values : "0" and "10".
Could you please tell me if such function already exists in Postgresql,
for one-dimensional objects, and also for multi-dimensional objects.

Thanks a lot in advance.
Virgile Beddok