  Why do we live in a (3+1) dimensional universe? In this paper, I review the
"relaxation principle" first introduced by Karch and Randall
(arXiv:hep-th/0506053v2) and generalize its ideas to an arbitrary (n+1)
dimensions. This is done by referring to the Friedman equation and the scaling
solution to derive the energy densities of the non-interacting and interacting
branes, and then formulating their dimensionalities. I also demonstrate that
the largest interacting d-brane in n spatial dimensions is always the
(n-2)-brane, and that such dimensionality constraint "relaxed" our universe
from nine dimensions to three dimensions.
