# Why does a reduction from a P-problem to an NP-complete problem not show that P=NP?

Consider the following problem, called BoxDepth:

Given a set of $n$ axis-aligned rectangles in the plane, how big is the largest subset of these rectangles that contain a common point?

Say we proved these two statements:

1. There is a a polynomial-time reduction from BoxDepth to MaxClique.

2. There is a polynomial-time algorithm for BoxDepth with $O(n^3)$ runtime.

Why don’t these two results imply that P=NP?

• Well, suppose there are $k'$ intersecting rectangles: what structure does that imply in the graph? Suppose there's a $k$-clique in the graph: what structure does that imply in the rectangles? – David Richerby Nov 14 '16 at 8:16
• Welcome to Computer Science! The title you have chosen is not well suited to representing your question; you can not "prove a reduction". Please take some time to improve it; we have collected some advice here. Thank you! – Raphael Nov 14 '16 at 17:13
• Please ask only one question per post. Since you already got an answer for c, I'm going to remove the others; feel free to repost them (one at a time). – Raphael Nov 14 '16 at 17:13
• You'll note that the new, better title pretty much answers the question. – Raphael Nov 14 '16 at 17:15