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Bjiirner, A. and C. Reutenauer, Rationality of the Mobius function of subword order, Theoretical Computer Science 98 (1992) 53-63. We prove the rationality of various noncommutative formal power series, whose coefficients are determined by the Mobius function or zeta function of the subword partial order of noncommutative monomials. We also give explicit expressions for the corresponding commutative generating functions.doi:10.1016/0304-3975(92)90379-t fatcat:izjan5fvovff3gf6sxwxpgtmgq