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In the paper "A triangle-free circle graph with chromatic number 5" from A.A. Ageev, I don't understand what the author mean with the following definition:

Somebody can explain it to me? (I'm not mathematician).

Thanks !

Link to the whole paper:

http://www.sciencedirect.com/science/article/pii/0012365X95003492

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# What does it mean? (Graph theory)

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