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Benign vs malicious ghosts in higher-derivative theories

Interacting theories with higher derivatives involve ghosts. They correspond to instabilities that display themselves at the classical level. We notice that comparatively "benign" mechanical higher-derivative systems exist where the classical vacuum is stable with respect to small perturbations and the problems appear only at the nonperturbative level. We argue that benign higher-derivative field theories exist which are stable with respect to small fluctuations with nonzero momenta. A particular example is the 6D N=2 higher-derivative SYM theory, which is finite and unitary at the perturbative level. The inflation-like instability with respect to small fluctuations of static modes is always present, however.