Digit sums

Interessant war es auch, drei aufeinanderfolgende Zahlen zu nehmen, von denen die größte durch drei teilbar sein musste, sie zu addieren und aus dem Ergebnis so lange die Quersumme zu bilden, bis eine einstellige Zahl übrig blieb. Diese Zahl war immer sechs.
— Child, Lee: Der Anhalter. München: Blanvalet, 2015; p. 73.

Jack Reacher’s at most tangentially to interpreting the sergeant’s reply related base ten factoid’s formal form is

\forall n\in\mathbb{N}^+:\mathrm{fds}_{10}\left(\sum\limits_{j=0}^2 3n-j\right) = 6,

where \mathrm{fds}_{10} represents the final digit sum in base ten.

A proof of the above claim together with the underlying digit sum results is presented in digit_sums.pdf (source: digit_sums.tex).

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