A sign pattern (matrix) is a matrix whose entries are from the set {+, ?, 0} and a sign vector is avector whose entries are from the set {+, ?, 0}. A sign pattern or sign vector is full if it does not contain anyzero entries. The minimum rank of a sign pattern matrix A is the minimum of the ranks of the real matriceswhose entries have signs equal to the corresponding entries of A. The notions of essential row sign changenumber and essential column sign change number are introduced for full sign patterns and condensed signpatterns. By inspecting the sign vectors realized by a list of real polynomials in one variable, a lower boundon the essential row and column sign change numbers is obtained. Using point-line confiurations on theplane, it is shown that even for full sign patterns with minimum rank 3, the essential row and column signchange numbers can differ greatly and can be much bigger than the minimum rank. Some open problemsconcerning square full sign patterns with large minimum ranks are discussed.
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