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# $k$-generalized Pell numbers which are repdigits in base $b$

## DOI

10.55730/1300-0098.3321

## Abstract

Let $k\geq 2$ be an integer and let $(P_{n}^{(k)})_{n\geq 2-k}$ be the $k$ -generalized Pell sequence defined by \begin{equation*} P_{n}^{(k)}=2P_{n-1}^{(k)}+P_{n-2}^{(k)}+...+P_{n-k}^{(k)} \end{equation*} for $n\geq 2$ with initial conditions \begin{equation*} P_{-(k-2)}^{(k)}=P_{-(k-3)}^{(k)}=\cdot \cdot \cdot =P_{-1}^{(k)}=P_{0}^{(k)}=0,P_{1}^{(k)}=1. \end{equation*} In this study, we deal with the Diophantine equation \begin{equation*} P_{n}^{(k)}=d\left( \frac{b^{m}-1}{b-1}\right) \end{equation*} in positive integers $n,m,k,b,d$ such that $m\geq 2,$ $2\leq b\leq 9$ and $1\leq d\leq b-1$. We show that the repdigits in the base $b$ in the $k-$ generalized Pell sequence, which have at least two digits, are the numbers \begin{eqnarray*} \ P_{7}^{(4)} &=&228=(444)_{7},\text{ }P_{4}^{(2)}=12=(22)_{5}\text{, }% P_{6}^{(2)}=70=(77)_{9}\text{;} \\ P_{4}^{(k)} &=&13=(111)_{3}\text{ } \end{eqnarray*} for $k\geq 3$ and \begin{equation*} P_{3}^{(k)}=5=(11)_{4} \end{equation*} for $k\geq 2.$

## Keywords

Repdigit, Fibonacci and Lucas numbers, Exponential Diophantine equations, linear forms in logarithms; Baker's method

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