@article {IOPORT.05014317,
    author       = {\D Zyli\'nski, Pawe\l},
    title        = {Orthogonal art galleries with holes: a coloring proof of Aggarwal's theorem.},
    year         = {2006},
    journal      = {The Electronic Journal of Combinatorics [electronic only]},
    volume       = {13},
    number       = {1},
    issn         = {1077-8926},
    pages        = {Research paper R20, 10 p.},
    publisher    = {Prof. Andr\'e K\"undgen, Deptartment of Mathematics, California State University San Marcos, San Marcos, CA},
    abstract     = {Summary: We prove that $\left\lfloor{n+h\over 4}\right\rfloor$ vertex guards are always sufficient to see the entire interior of an $n$-vertex orthogonal polygon $P$ with an arbitrary number $h$ of holes provided that there exists a quadrilateralization whose dual graph is a cactus. Our proof is based upon $4$-coloring of a quadrilateralization graph, and it is similar to that of {\it J. Kahn} and others [SIAM J. Algebraic Discrete Methods 4, 194--206 (1983; Zbl 0533.05021)] for orthogonal polygons without holes. Consequently, we provide an alternate proof of Aggarwal's theorem asserting that $\left\lfloor{n+h\over 4}\right\rfloor$ vertex guards always suffice to cover any $n$-vertex orthogonal polygon with $h \le 2$ holes.},
    identifier   = {05014317},
}