Articles which reference this paper are Luz M. DeAlba, Jason Grout, In-Jae Kim, Steve Kirkland, Judith J. McDonald and Amy Yielding Minimum rank of powers of trees. Electron. J. Linear Algebra 23 (2012) 151-163. Travis Peters Positive semidefinite maximum nullity and zero forcing number. Electron. J. Linear Algebra 23 (2012) pp. 815-830.