Let ${\left(L_n\right)}_{n\ge 0}$ be the Lucas sequence. We show that the Diophantine equation $L_n-L_m=M_k$ has only the nonnegative integer solutions $(n,m,k)=(2,0,1)$, $(3,1,2)$, $(3,2,1)$, $(4,3,2)$, $(5,3,3)$, $(6,2,4)$, $(6,5,3)$ where $M_k=2^k-1$ is the $k$th Mersenne number and $n>m$.

We consider the Tribonacci sequence $T:={T_n}_{n\ge 0}$ given by T₀ = 0, T₁ = T₂ = 1 and $T_{n+3}=T_{n+2}+T_{n+1}+T_n$ for all n ≥ 0, and we find all triples of Tribonacci numbers which are multiplicatively dependent.