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[parent] sequences $b^{2n}-1$ and $b^{2n-1}+1$ are divisible by $b+1$ (Derivation)

Consider the alternating geometric finite series

$\displaystyle S_{m+1}(\mu)=\sum_{i=0}^m(-1)^{i+\mu}b^i,$ (1)

where $\mu=1,2$ and $b\geq 2$ an integer. Multiplying (1) by $-b$ and subtracting from it
$\displaystyle (b+1)S_{m+1}(\mu)=\sum_{i=0}^m(-1)^{i+\mu}b^i-\sum_{i=0}^m(-1)^{i+1+\mu}b^{i+1}$    

and by elemental manipulations, we obtain
$\displaystyle S_{m+1}(\mu)=\frac{(-1)^\mu[1-(-1)^{m+1}b^{m+1}]}{b+1}= \sum_{i=0}^m(-1)^{i+\mu}b^i.$ (2)

Let $\mu=1$ , $m=2n-1$ . Then
$\displaystyle \frac{b^{2n}-1}{b+1}=-\sum_{i=0}^{2n-1}(-1)^ib^i.$ (3)

Likewise, for $\mu=2$ , $m=2n-2$
$\displaystyle \frac{b^{2n-1}+1}{b+1}=\sum_{i=0}^{2n-2}(-1)^ib^i,$ (4)

as desired.

Palindromic numbers of even length

As an application of above sequences, let us consider an even palindromic number (EPN) of arbitrary length $2n$ which can be expressed in any base $b$ as
$\displaystyle (EPN)_n=\sum_{k=0}^{n-1}b_k(b^{2n-1-k}+b^k)= \sum_{k=0}^{n-1}\frac{b_k}{b^k}[(b^{2n-1}+1)+(b^{2k}-1)],$ (5)

where $0\leq b_k\leq b-1$ .It is clear, from (3) and (4), that $(EPN)_n$ is divisible by $b+1$ . Indeed this one can be given by
$\displaystyle (EPN)_n=(b+1)\sum_{k=0}^{n-1}\sum_{j=0}^{2(n-1-k)}b_k(-1)^jb^{k+j}.$ (6)




"sequences $b^{2n}-1$ and $b^{2n-1}+1$ are divisible by $b+1$" is owned by perucho.
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Cross-references: divisible, clear, base, length, palindromic number, even, sequences, application, integer, series, finite, alternating

This is version 3 of sequences $b^{2n}-1$ and $b^{2n-1}+1$ are divisible by $b+1$, born on 2006-09-11, modified 2006-09-14.
Object id is 8340, canonical name is SequencesB2n1AndB2n11AreDivisibleByB1.
Accessed 850 times total.

Classification:
AMS MSC11A63 (Number theory :: Elementary number theory :: Radix representation; digital problems)

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