If your vector is guaranteed to be in sorted order, as in your example, then
max(find(a<k))
and
min(find(a>k))
will give the two limits.
You might want to make one of those include the equality, in case it might be exactly equal to the border. Also, you really only need to calculate, say, the lower limit, and then the other limit is just the next index.
Finally, you might need to consider what to do if all values in the vector are above (or below) the value of k.
