# Rigging Elections

## Contents

## Problem description

In a country not very far away, the party of the duke Sark Mevo has finally beaten the ruling coalition headed by the princess Reguel Tekris. Mevo wishes to consolidate his position in the capital, the fourteen quarters of which need to be grouped to electoral districts. A schematic map of the capital is given in the figure below. The quarters are numbered from 1 to 14. The two other numbers are the forecast number of favorable votes for Mevo and the total number of electors per quarter. All electors must vote and the winner needs to have the absolute majority. A valid electoral district is formed by several adjacent quarters and must have between 30,000 and 100,000 voters. Two quarters that touch each other just at a point like 12 and 13 are not considered adjacent. Electoral districts consisting of a single quarter are permitted if it has at least 50,000 voters. Nevertheless, Mevo may not decently define an electoral district solely with quarter 10, since this contains his residence. Determine a partitioning into five electoral districts that maximizes the number of seats for Mevo. Should this cause any difficulties, try a partitioning into six districts. Snirp, the mathematical jester, suggests Mevo uses Mathematical Programming...

+-------------+--+------+-------------------------------+ | 1 | | 6 | 7 12000/30000 | | 17500/30000 | | 9000/+---------------+---------------+ +-------------+ |40000 | 8 | | | 2 | | | 10000/30000 | 9 | | 15000/50000 | +------+-------+-------+ 26000/40000 | +-------------+ 5 | | 11 | | | 3 | 18000/ | 10 | 2500/ +---------------+ | 14200/20000 | 20000 | 34000/| 10000 | 12 27000/60000| +-------------+---------+ 60000 +-------+---------------+ | 4 | | 13 | 14 | | 42000/70000 | | 29000/| 15000/40000 | + | | 40000 | | +-----------------------+-------+-------+---------------+

Map of the capital and its quarters. Legend: quarter number, supporters/electorate

## Variables

in/out Quarter in(i) borders to quarter out(i) population Population of each quarter votes Supporters er quarter minpop Min size of an electoral district maxpop Max size of an electoral district minsingle Min size of a quarter to be a district districtsnum Number of districts wanted illegalsingle This quarter may not be single districts The possible partition of the quarters into desired number of districts.

## Reference

Applications of optimization... Gueret, Prins, Seveaux

% Marcus Edvall, Tomlab Optimization Inc, E-mail: tomlab@tomopt.com % Copyright (c) 2005-2009 by Tomlab Optimization Inc., $Release: 7.2.0$ % Written Oct 7, 2005. Last modified Apr 8, 2009.

## Problem setup

function tomsym_riggingelections in = [1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 8 9 9 10 10 11 11 12 13]'; out = [2 5 3 5 4 5 5 10 6 10 7 8 8 9 9 10 11 11 12 11 13 12 13 14 14]'; population = [30 50 20 70 20 40 30 30 40 60 10 60 40 40]'; votes = [17.5 15 14.2 42 18 9 12 10 26 34 2.5 27 29 15]'; minpop = 30; maxpop = 100; minsingle = 50; districtsnum = 6; illegalsingle = 10; districts = []; rowlen = length(population); maxq = rowlen; for q=1:maxq if (population(q) >= minsingle & q ~= illegalsingle) districts = [districts; zeros(1,maxq)]; districts(end, q) = 1; end idx = find(in == q); for i = 1:length(idx) p = out(idx(i)); if (population(q) + population(p) <= maxpop) if (population(q) + population(p) >= minpop) districts = addNeighbor(districts, q, p, rowlen); idx2 = find(in == p); for j = 1:length(idx2) r = out(idx2(j)); if (population(q) + population(p) + population(r) <= maxpop) if (population(q) + population(p) + population(r) >= minpop) districts = addNeighbor(districts, q, p, rowlen, r); end end end end end end end n = size(districts, 1); %possible districts % Calculate majority d = zeros(n,1); for i=1:n idx = find(districts(i,:) == 1); if sum(votes(idx))/sum(population(idx)) > 0.5 d(i,1) = 1; end end choose = tom('choose',n,1,'int'); % All variables are binary bnds = {0 <= choose <= 1}; % Districts chosen con1 = {sum(choose) == districtsnum}; % Quarter only once con2 = {districts'*choose == 1}; % Objective objective = -d'*choose; constraints = {bnds, con1, con2}; options = struct; options.solver = 'cplex'; options.name = 'Rigging Elections'; sol = ezsolve(objective,constraints,[],options); sol.districts = districts; PriLev = 1; if PriLev > 0 temp = sol.districts(find(sol.choose),:); disp('to rig the elections:') for d = 1:size(temp,1), qs = find(temp(d,:)); disp([' merge the quarters ' num2str(qs) ' into a district.']) end end function districts = addNeighbor(districts, q, p, rowlen, r) if nargin <5 districts = [districts; zeros(1,rowlen)]; districts(end, [q, p]) = [1 1]; else districts = [districts; zeros(1,rowlen)]; districts(end, [q, p, r]) = [1 1 1]; end % MODIFICATION LOG % % 051205 med Created % 060118 per Added documentation % 060125 per Moved disp to end % 090325 med Converted to tomSym

Problem type appears to be: mip ===== * * * =================================================================== * * * TOMLAB - Tomlab Optimization Inc. Development license 999001. Valid to 2010-02-05 ===================================================================================== Problem: --- 1: Rigging Elections f_k -5.000000000000000000 f(x_0) 0.000000000000000000 Solver: CPLEX. EXIT=0. INFORM=101. CPLEX Branch-and-Cut MIP solver Optimal integer solution found FuncEv 10 to rig the elections: merge the quarters 1 2 5 into a district. merge the quarters 3 4 into a district. merge the quarters 6 7 8 into a district. merge the quarters 9 12 into a district. merge the quarters 10 11 into a district. merge the quarters 13 14 into a district.