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Lecture #1 [Supowit and Reingold 1983] The complexity of drawing trees nicely
This paper shows that one can use LP-based methods to minimize the width of a "balanced-layered" drawing of tree, but if one desires a grid drawing the problem becomes NP-hard.
Click http://link.springer.com/article/10.1007%2FBF00289576 link to open resource.