  We find explicitly all rational solutions of the title equation for all
integers $k$ in the range $|k|\leq 50$ except for $k=-47,-39$. For the
solution, a variety of methods is applied, which, depending on $k$, may range
from elementary, such as divisibility and congruence considerations, to
elliptic Chabauty techniques and highly technical computations in algebraic
number fields, or a combination thereof. For certain sets of values of $k$ we
can propose a more or less uniform method of solution, which might be applied
successfully for quite a number of cases of $k$, even beyond the above range.
It turns out, however, that in the range considered, six really challenging
cases have to be dealt with individually, namely $k = 15,43,-11,-15,-39,-47$.
More than half of the paper is devoted to the solution of the title equation
for the first four of these values. For the last two values the solution of the
equation, at present, has resisted all our efforts. The case with these six
values of $k$ shows that one cannot expect a general method of solution which
could be applied, even in principle, for {\em every} value of $k$. A summary of
our results is shown at the end of the paper.
