Hello,

I just finished watching a short video on youtube explaining the concepts of increasing, decreasing, and constant functions. I understand such concepts now, but what was most notable in the video is when the instructor remarked that intervals are named using the

x-values only? To my knowledge, an interval is a portion of real numbers on the real number line (is that correct?). And so, could you not have a interval with respect to the

y-axis--since it is a real number line?

Thank you