Backstory
I started playing a mobile based FPS shooting game which among different modes of gameplay has the freeforall deathmatch. A freeforall(F4A) deathmatch is where every player fights against everyone else and the first player to reach a set point (number of kills) wins the match.
Let there be \(N\) players in a F4A deathmatch, indexed by small latin indices (\(i,j\)). When the game ends, we can calculate what I call the KillDeath (KD) matrix.
KDmatrix is an \(N\times N\) matrix where \(k_{ij}\) means number of times player \(i\) kills player \(j\). Then, total kills by player \(i\) becomes, \(K_i = \sum_{j} k_{ij}\) and total deaths by player \(i\) becomes, \(D_i = \sum_{j} k_{ji}\). Naturally, \(k_{ii} = 0\), when player kills himself or herself (usually happens when you fail to throw a grenade or use the grenade launcher too close) no one gets any point.
Problem started off as estimating \(D_i\) given \(K_i\) which soon proved to be impossible. But, straining my two braincells further got me pose the problem like this:
Problem statement
Given sum of rows and sum of columns of a square matrix with all diagonal elements as zero, give an estimate of square matrix.
I call this the KD problem.
Degree of freedom
An \(N \times N\) square matrix with all diagonal elements zero has \(^N P_2\) Degrees of Freedom (DOF).
The rowsums and columnsums each contribute \(N\) simutaneous linear equations.
So, effective DOF is \(^NP_2  2N\).
Trivial case
\(N=2\) is a trivial case.
\[KD = \begin{bmatrix} 0 & k_{12} \\ k_{21} & 0 \end{bmatrix}\]Rowsum which is \([k_{12}, k_{21}]\) or columnsum which is \([k_{21}, k_{12}]\) directly gives us the answer.
Base case
\(N=3\) proves to be an enlighting case as \(^3P_2  2\cdot3 = 0\), meaning effective DOF is \(0\).
Let us have \(KD\)matrix as follows:
\[KD = \begin{bmatrix} 0 & a & b \\ c & 0 & d \\ e & f & 0 \end{bmatrix}\]Rowsum is \([ a+b, c+d, e+f]^T\) and columnsum is \([c+e, a+f, b+d]^T\). Also, we can define \(B\) vector as \([a+b, c+d, e+f, c+e, a+f, b+d]^T\), which is rowsum after columnsum concatenated.
Populating our \(X\) vector as \([a,b,c,d,e,f]^T\). We can define/write \(A\) matrix as
\[A = \begin{bmatrix} 1 & 1 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 1 & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 & 1 \\ 0 & 0 & 1 & 0 & 1 & 0 \\ 1 & 0 & 0 & 0 & 0 & 1 \\ 0 & 1 & 0 & 1 & 0 & 0 \end{bmatrix}\]Now, we can write a system of equations as \(AX = B\)
Unfortunately, it is not that simple. \(det(A)\) vanishes. So, we have to apply a pseudoinverse technique.
Pseudoinverse
These notes proved to be very helpful. PseudoInverse
Any square matrix \(A\) has an unique pseudoinverse \(A^+\).
General solution of \(Ax=b\) is \(x = A^+ b + (I  A^+ A) y\) where \(y\) brings in the additional DOFs by being an arbitrary vector with suitable dimension (\(^NP_2\)).
General case
In a general \(N\) case, effective DOF is non zero and hence it is an underdetermined system of linear equations. All the more reason to use pseudoinverse.
\(A\) matrix can be generated in chucks of \(N1\) vectors.
To deal with rowsums, we put \(N1\) ones mutually exclusively in a diagonal fashion.
Dealing with columnsums is a bit tricky. The diagonal chunks are zeros to eliminate corresponding rows from summation. In the upper triangle portion, we populate chunks starting from \((10...0)\) to \((00..01)\) moving \(1\) from leftmost position to rightmost position everytime we come across a principle diagonal element.
Refer to this snippet of python code:
import numpy as np
# handy variable
n1 = n  1
# N P 2
p = n * n1
# twice of n, this is the number of equations
n2 = 2*n
# this is the A matrix
ret = np.zeros ((n2,p), dtype=DTYPE)
# rows dealing with the rowsums section
# kill section
for i in range(n):
j = i*n1
ret[i,j:(j+n1)] = 1
# rows dealing with the columnsums section
# death section
def ones(n,k=1):
"""Creates a 1D onehot encoded array"""
r = np.zeros (n, dtype=DTYPE)
if k != 1:
r[k] = 1
return r
for i in range(n,n2):
ii = i  n
in1 = ii*n1
for j in range(0,p,n1):
if in1 == j:
# zeros
ret[i,j:(j+n1)] = ones(n1)
elif j > in1:
ret[i,j:(j+n1)] = ones(n1,ii)
elif j < in1:
ret[i,j:(j+n1)] = ones(n1,ii1)
Test case
Imagine a test with given \(KD\)matrix as
\[\begin{bmatrix} 0 & 4 & 0 & 1 & 6 \\ 0 & 0 & 0 & 0 & 2 \\ 1 & 0 & 0 & 1 & 1 \\ 5 & 3 & 4 & 0 & 8 \\ 0 & 4 & 1 & 0 & 0 \end{bmatrix}\]Then, rowsums (or Kills) is \([10,2,3,20,5]^T\) and columnsums (or Deaths) is \([6,11,5,2,17]^T\).
Using pseudoinverse and trivial \(y\) vector, we get,
\[\begin{bmatrix} 0.0 & 2.98 & 1.45 & 1.78 & 4.78 \\ 0.19 & 0.0 & 0.61 & 0.28 & 2.71 \\ 0.04 & 0.78 & 0.0 & 0.41 & 2.58 \\ 4.38 & 5.11 & 3.58 & 0.0 & 6.91 \\ 1.38 & 2.11 & 0.58 & 0.91 & 0.0 \end{bmatrix}\]The MSE turns out to be \(0.998\) which is pretty good.
Future things to care
 Smart way to use the arbitrary \(y\) which better constraints the \(KD\)matrix.

Find MSE as a function of \(y\) or \(\lVert y \rVert\).
 Optimize the \(y\) by treating it as an regularized problem.
 Understand the residual with \(y\) as a parameter.


Relax traceless condition.
 Generalized matrix and not just a square matrix.