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Mazdak Farrokhzad edited proof b.tex
about 10 years ago
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\subsection{b)}
Consider
running the
proof for task a), above algorithm, but
now with
$I_1$ as the first interval ending after $R$ chosen to be an arbitrary
starting point $P$. If $start(I_1) \geq P$, the case is equivalent with
above, and it is possible to find the optimal solution. If $start(I_1) < P$, it
is possible that there exists ray,
instead of one
or more intervals $I_n$ such that
$start(I_1) \mod{2\pi} \leq end(I_n) < P$. If which does not intersect any of the
intervals. The algorithm
adds will
then select the
interval $I_1$, which is the interval which ends first after $R$. A
ray
$R_n$ $R_1$ with
an angle
in the $end(I_1)$ will be chosen to intersect $I_1$. Since
$angle(R) \leq end(I_1) = angle(R_1)$, and $I_1$ is chosen such that for any all intervals
$I_k, end(I_k) \geq angle(R) \implies end(I_k) >= end(I_1)$, any interval
$[start(I_1), end(I_n)]$, it intersected by $R$
is
possible also intersected by $R_1$.
Now, consider removing all intervals intersected by $R_1$. This means that
the ray $R_1$
added at
$end(I_1)$ will
not be
needed, needed any more, and
could possibly can be removed.
The rest Since any interval intersected by $R$ is
also intersected by $R_1$, and all intervals intersected by $R_1$ have been removed,
$R$ no longer intersects any interval. Thus, by removing the first ray and the
intervals which it intersects, the problem is converted to the special case which
we proved that our algorithm can find the optimal solution for earlier.
Since the algorithm ignores any interval which has already been intersected by an
earlier ray, the step of
choosing the
rays
will always be needed, since first ray is equivalent to removing
any the
intervals intersected by it. Thus, the remaining part of
them the problem will be solved
with a solution which would
cause at least one circle have been optimal if the intervals intersected by the
first ray were not
present.
Since the smaller problem is a subset of the larger problem, and any interval
intersected by a ray in the optimal solution to the smaller problem still needs to
be intersected
any more. This means that by a ray in the
result might have larger problem, it is impossible to solve the
larger problem with fewer rays than the smaller problem. Since our algorithm uses
only one more ray
to solve the larger problem, it is impossible for a solution
which uses more than
optimal, but not more. one ray less than our solution to exist, since such a
solution would have fewer rays than needed to solve the smaller problem, which
is a contradiction.