Pillai's equation between k-generalized Pell numbers and Tribonacci numbers

Source abstract

For an integer $k\ge2$, let $(P_n^{(k)})_{n\ge0}$ be the $k$-generalized Pell sequence and $(T_m)_{m\ge1}$ the Tribonacci sequence. We determine, for every $k\ge2$, all integers $c$ having at least two representations $c=P_n^{(k)}-T_m$ with $m\ge2$. The starting point is an identity of Kılıç: $P_n^{(k)}=F_{2n+1}$ for $0\le n\le k$. In the range $n\le k$ the problem therefore becomes, for all $k$ at once, Pillai's equation between odd-indexed Fibonacci numbers and Tribonacci numbers, which has exactly six solutions. In the range $n>k$ we solve the problem for $k=2$ and $k=3$ using the exact heights of the Binet coefficients; for $k\ge4$ we prove a bound for $n$ polynomial in $k$. We also use the fact that $P_n^{(k)}$ is extremely close to $F_{2n+1}$ when $k$ is large to show that $k\le414$, and treat the remaining values one by one. For $k\ge4$ there are, besides the solutions common to all $k$, exactly four further solutions, for $k=4$, $5$ and $9$. The proofs combine Matveev's theorem with the reduction method of Dujella and Pethő.