TITLE: Existence/Uniqueness of Nonnegative Solutions of Linear Systems of Equations
QUESTION [5 upvotes]: Suppose we have an $m$x$n$ matrix $A$, with $m\lt n$, and an $m$x$1$ vector $b$. Are there existence and uniqueness conditions characterizing nonnegative solutions of the system of linear equations $Ax=b$? i.e. When is there an $x\geq 0$ such that $Ax$=$b$?
I'm sure it is a very well-known result I'm after here but I can't seem to find the answer easily. Any references would be helpful. This (http://www.jstor.org/pss/1968384) looks related but it is not free.

REPLY [5 votes]: The following work seems to be exactly what you want.
M. Wang, and A. Tang. 
Conditions for a Unique Non-negative Solution to an Underdetermined System.
in Proc. of Allerton Conference , Monticello, Illinois, Sep. 2009.  
Here is the link:
http://networks.ece.cornell.edu/meng/pub/Allerton09.pdf