The production of a Higgs boson in association with a top quark pair ($\mathrm{t}\overline{\mathrm{t}}\mathrm{H}$) is measured in final states with electrons, muons and hadronically decaying $\tau$ leptons. Events are selected from a sample of proton-proton collisions at a \sqrt{s} = 13~\mathrm{TeV} center-of-mass energy, recorded by the CMS experiment in 2017 and corresponding to an integrated luminosity of 41.5~\mathrm{fb}^{-1}. Machine learning and matrix element methods are used to optimize the analysis sensitivity. The observed (expected) $\mathrm{t}\overline{\mathrm{t}}\mathrm{H}$ production rate is $0.75^{+0.46}_{-0.43}$ ($1.00^{+0.39}_{-0.35}$) times the expected rate in the standard model, which corresponds to an observed (expected) significance of $1.7\sigma$ ($2.9\sigma$). In addition, these results are combined with those previously obtained from the data set collected in 2016 and corresponding to an integrated luminosity of 35.9~\mathrm{fb}^{-1}. The observed (expected) signal rate for the combined fit is $0.96^{+0.34}_{-0.31}$ ($1.00^{+0.30}_{-0.27}$) times the expected rate in the standard model, which corresponds to an observed (expected) significance of $3.2\sigma$ ($4.0\sigma$).

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