In their famous book, Garey and Johnson, write a comment that the maximum independent set problem, in cubic planar graphs is NP-complete(page 194 of the book). They say this is by a transformation from vertex cover and for vertex cover (I suppose in cubic planar graphs) they cite the paper of Garey, Johnson and Stockmeyer 1976.

When I looked at their DBLP entry, there is a single paper that matches the description of the mentioned paper:

Some Simplified NP-complete Graph Problems.

Having checked the paper, the reduction for vertex cover in graphs of maximum degree 3 is for general graphs, not planar graphs. For planar graphs, they only have a reduction that shows hardness on planar graphs of maximum degree 6, not 3 (already the gadget has vertices of degree 6).

I'm wondering if I'm missing something here since there are consequent papers building upon the aforementioned comment of the book.


2 Answers 2


A complete NP-completeness proof for this problem is given right after Theorem 4.1 in the following paper.

Bojan Mohar:
"Face Covers and the Genus Problem for Apex Graphs"
Journal of Combinatorial Theory, Series B 82, 102-117 (2001)


Actually, there is a simple gadget to remove vertices of degree larger than three. See, e.g., the answer here. Note that this gadget keeps planarity.

  • $\begingroup$ Mohar did similar to this (see the other answer to this post), by now it is a standard technique, back then I have no idea. $\endgroup$
    – Saeed
    Commented Apr 23, 2020 at 6:47

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