  Motivated by results about "untangling" closed curves on hyperbolic surfaces,
Gupta and Kapovich introduced the primitivity and simplicity index functions
for finitely generated free groups, $d_{prim}(g;F_N)$ and $d_{simp}(g;F_N)$,
where $1\ne g\in F_N$, and obtained some upper and lower bounds for these
functions. In this paper, we study the behavior of the sequence
$d_{prim}(a^nb^n; F(a,b))$ as $n\to\infty$. Answering a question of Kapovich,
we prove that this sequence is unbounded and that for $n_i=lcm(1,2,\dots,i)$,
we have $|d_{prim}(a^{n_i}b^{n_i}; F(a,b))-\log(n_i)|\le const$. By contrast,
we show that for all $n\ge 2$, one has $d_{simp}(a^nb^n; F(a,b))=2$. In
addition to topological and group-theoretic arguments, number-theoretic
considerations, particularly the use of asymptotic properties of the second
Chebyshev function, turn out to play a key role in the proofs.
