# Maximize the area of a radar chart

People use radar charts to present their ability in different skills.

Like all self presentations, it need to make the person "look good". A chart with larger area looks better than the ones with less area. The remaining of the article take about how to find a radar chart with maximum area.

In the image above, the left side has less area than the right.

Given the sequence of data (a_1,c_1),\ldots,(a_n,c_n), where a_i is the value, a positive number, c_i is a coordinate(in the image, the coordinates would be "combinatorics", "algorithms", etc.). How can we permute them, such that the data plotted on the radio chart is maximized?

The area of of any single section of the radio chart is ab \frac{\cos \theta}{2}, where \theta = \frac{2\pi}{n}, and a,b are the value for two adjacent coordinates. The area of the entire shape is the sum.

Once we can maximize \sum_{i=1}^n a_{\pi(i)} a_{\pi(i+1)}, we are done.

If x\geq a\geq b \geq c \geq d\geq 0, then ax+bx - ab \geq cx+dx-cd.

Let's consider what is required to make sure the property works. \displaystyle \begin{aligned} (a+b)x-ab &\geq (c+d)x-cd\\ x&\geq \frac{ab-cd}{a-c+b-d}\\ x&\geq \frac{ab-bc+bc-cd}{a-c+b-d}\\ x&\geq \frac{b(a-c)+c(b-d)}{a-c+b-d}\\ x&\geq \frac{b(a-c+b-d)}{a-c+b-d}\\ x&\geq b \end{aligned} The lemma is true, as x\geq b is part of the hypothesis for x.

Let a_1,\ldots,a_n be a sequence of positive numbers, such that a_1 \leq \ldots \leq a_n.

Let \pi be the following:

- \pi(n+1) = 1.
- \pi(1),\ldots,\pi(n) = 1, 3, \ldots, n-1, n, n-2, \ldots, 4, 2 if n is even
- \pi(1),\ldots,\pi(n) = 1, 3, \ldots, n-2, n, n-1, \ldots, 4, 2 if n is odd.

\pi maximizes \displaystyle \sum_{i=1}^n a_{\pi(i)}a_{\pi(i+1)}

Proof by induction.

*Base case:* Base case for n=1,2 are trivial.

*Inductive Step:* Consider it is true for n-1, we want to show it is true for n.

Consider we already obtained a sequence from a_1 to a_{n-1}, and we want to insert a_n somewhere. Clearly a_n need to be placed in a position that maximizes its contribution. If we can insert it between a_{n-1} and a_{n-2}, then we get a contribution of a_{n-1}a_n+a_{n-2}a_n-a_{n-1}a_{n-2}. By the lemma, it will give us the maximum contribution.

If we can get the maximum contribution (over all possible configurations) to a maximum configuration for a_1 to a_{n-1}, then we are done. The inductive hypothesis show the maximum configuration have adjacent a_{n-1} and a_{n-2}, therefore this shows the maximum configuration is where we insert a_n in between a_{n-1} and a_{n-2} in the previous maximum configuration.

This implies a simple O(n \log n) algorithm. Find the permutation that sort the input sequence by value, then composes it with permutation \pi.

Actually this problem is a special case of the TSP problem on a product matrix. A matrix is called a product matrix if M_{i,j} = a_ib_j for vectors (a_1,\ldots,a_n) and (b_1,\ldots,b_n). It is solvable in polynomial time if M is a symmetric matrix [1].

# References

[1] R.E. Burkard, V.G. Deῐneko, R. van Dal, J.A.A. van der Veen, G.J. Woeginger, **Well-solvable special cases of the traveling salesman problem: A survey**, SIAM Review. 40 (1998) pp. 496–546.