Let Hn be the n-th harmonic number and let vn be its denominator. It is known that vn is even for every integer
Boyd, D. W., A p-adic study of the partial sums of the harmonic series, Exp. Math., 3(4 ) (1994), 287–302.
Duncan, R. L., Note on the initial digit problem, Fibonacci Quart ., 7(5 ) (1969), 474–475.
Eswarathasan, A. and Levine, E., p-integral harmonic sums, Discrete Math ., 91(3 ) (1991), 249–257.B.-L. WU and X.-H. YAN: Some Properties of Harmonic Numbers.
LEONETTI, P. and Sanna, C., On the p-adic valuation of Stirling numbers of the first kind, Acta Math. Hungar., 151 (2017), 217–231.
SANNA, C., On the p-adic valuation of harmonic numbers, J. Number Theory, 166 (2016), 41–46.
SHIU, P., The denominators of harmonic numbers, arXiv:1607.02863v1.
Wu, B. L. and Chen, Y. G., On certain properties of harmonic numbers, J. Number Theory, 175 (2017), 66–86.
Wu, B. L. and Chen, Y. G., On the denominators of harmonic numbers, C. R. Acad. Sci. Paris, Ser I 356 (2018), 129–132.
Wu, B. L. and Chen, Y. G., On the denominators of harmonic numbers, II, J. Number Theory, 200 (2019), 397–406.