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Abstract
We answer the question in the title in the negative by providing four proofs.
Acknowledgment
Our thanks go to the two referees and also to the editor, for their careful reading of our manuscript and for the many improvements they suggested.
Additional information
Notes on contributors
Martin Klazar
MARTIN KLAZAR teaches mathematics to students of computer science at Charles University in Prague. He enjoys walks, sometimes even jogging, on hill Vítkov near his residence.
Richard Horský
RICHARD HORSKÝ teaches mathematics at Prague University of Economics and Business. He enjoys random walks in forests near his house in Březová-Oleško.