We investigate the complexity of LTL learning, which consists in deciding given a finite set of positive ultimately periodic words, a finite set of negative ultimately periodic words, and a bound B given in unary, if there is an LTL-formula of size less than or equal to B that all positive words satisfy and that all negative violate. We prove that this decision problem is NP-hard. We then use this result to show that CTL learning is also NP-hard. CTL learning is similar to LTL learning except that words are replaced by finite Kripke structures and we look for the existence of CTL formulae.
