[1] F. T. Adams-Watters and F. Ruskey, Generating functions for the digital sum and other digit counting sequences, J. Integer Seq., 12 (2009).
[2] J,-P. Allouche and J. Shallit, Sums of digits and the Hurwitz zeta function, Analytic number theory (Tokyo, 1988), Lecture Notes in Math., Springer, Berlin, 1990 19–30.
[3] L. E. Bush, An asymptotic formula for the average sum of the digits of integers, Amer. Math. Monthly, 47 (1940) 154–156.
[4] T. Cai, On 2-Niven numbers and 3-Niven numbers, Fibonacci Quart., 34 no. 2 (1996) 118–120.
[5] C. Cooper and R. E. Kennedy, On consecutive Niven numbers, Fibonacci Quart., 31 no. 2 (1993) 146–151.
[6] E. Estrada and L. Pogliani, A new integer sequence based on the sum of the digits of integers, Kragujevac J. Science, 30 (2008) 45–50.
[7] E. Estrada and P. Pereira-Ramos, Spatial “Artistic” Networks: From Deconstructing Integer-Functions to Visual Arts. Complexity, Vol. 2018, Article ID 9893867.
[8] E. Estrada, Integer-digit functions: an example of math-art integration, Math. Intelligencer, 40 no. 1 (2018) 73–78.
[9] H. G. Grundman, Sequences of consecutive n-Niven numbers, Fibonacci Quart., 32 no. 2 (1994) 174–175.
[10] G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, Oxford University Press, 5th ed., 1980.
[11] D. R. Kaprekar, Problems involving reversal of digits, Scripta Math., 19 (1953) 81–82.
[12] K. H. Rosen, Discrete Mathematics and its Applications, 7th Edition, McGraw-Hill, New York, 2012, 269–270.
[13] M. Schneider and R. Schneider, Digit sums and generating functions, Ramanujan J., 52 no. 2 (2020) 291–302.
[14] J. O. Shallit, On infinite products associated with sums of digits, J. Number Theory, 21 no. 2 (1985) 128–134.
[15] N. J. A. Sloane, The On-Line Encyclopedia of Integer Sequences (OEIS), http://oeis.org.
[16] A. J. Van der Poorten, An introduction to continued fractions, Diophantine analysis (Kensington, 1985), London Math. Soc. Lecture Note Ser., 109, Cambridge Univ. Press, Cambridge, 1986 99–138.
[17] C. Vignat and T. Wakhare, Finite generating functions for the sum-of-digits sequence, Ramanujan J., 50 no. 3 (2019) 639–684.