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Integer sequences of the form $\lfloor n^c\rfloor$, where 1 < c < 2, can be locally approximated by sequences of the form ⌊nα+β⌋ in a very good way. Following this approach, we are led to an estimate of the difference ${\sum }_{n\le x}\phi \left(\lfloor n^c\rfloor \right)-1/c{\sum }_{n\le x^c}\phi \left(n\right)n^{1/c-1}$, which measures the deviation of the mean value of φ on the subsequence $\lfloor n^c\rfloor$ from the expected value, by an expression involving exponential sums. As an application we prove that for 1 < c ≤ 1.42 the subsequence of the Thue-Morse sequence indexed by $\lfloor n^c\rfloor$ attains both of its values with asymptotic...