Fibonacci, Dirichlet, and Gauss in a single sum

We study the fractional-part sums $\sum_{k=1}^{n}\{F_n/F_k\}$, where $F_n$ is the $n$th Fibonacci number.

Their asymptotic behavior depends on the parity of $n$.

For odd $n$, the remainder is expressed in terms of the Gauss circle error term.

For even $n$, it is expressed in terms of the Dirichlet divisor error term.

Thus determining the optimal remainder exponent for the odd Fibonacci sums is equivalent to the Gauss circle problem, while the corresponding question for the even sums is equivalent to the Dirichlet divisor problem.

We also prove analogous formulas for a family of second-order recurrences, including the Lucas sequence, for which the roles of the two parities are exchanged.