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%% Author: Jun He
%% Date: 2017.05.11
% designed by Jun He on 11 May 2015
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\title{{\Huge An Initial Analysis of Approximation Error for Evolutionary Algorithms}} % Poster title
\author{\Large Jun He \inst{1} \and Yuren Zhou \inst{2} \and Guangming Li \inst{3}}
\institute{\normalsize \inst{1}Aberystwyth University, UK \,\, \inst{2} Sun Yat-sen University, China\,\, \inst{3} Shenzhen Institute of Information Technology, China}
%% own commands
\newcommand{\opt}{\mathrm{max}}
\newcommand*{\defeq }{:=}
\begin{document}
\begin{frame}[t]
\maketitle
\begin{columns}[t]
\begin{column}{0.45\paperwidth}
%%%%%%%%%%% Start of Column 1 %%%%%%%%%%
\begin{greenblock}{Motivation}
This work aims at rigorously analyzing the approximation error of evolutionary algorithms (EAs).
\end{greenblock}
\begin{greenblock}{Background}
Consider an EA for solving a maximization problem:
\begin{align}
\max f(x), \mbox{ subject to } x \in \mathcal{S}.
\end{align}
Let $f_{\opt}$ denote the fitness of the optimal solution and $F_t$ the expected fitness of the best solution found in the $t$th generation.
\begin{definition}{Definitions}
\begin{itemize}
\item The \textbf{approximation error} of the EA in the $t$th generation is
\begin{equation}
E_t:=f_{\opt}-F_t.
\end{equation}
\item If some positive constants
$\alpha$ and $\beta$ exist with
\begin{align}
\lim_{t \to +\infty} \frac{E_t}{(E_{t-1})^{\alpha}} =\beta,
\end{align} then $\{E_t; t=0,1, \cdots\}$ is called to \textbf{converge to $0$ in the order $\alpha$}, with \textbf{asymptotic error constant $\beta$}~\cite{burden2015numerical,gautschi2011numerical}.
\end{itemize}
\end{definition}
\end{greenblock}
\begin{greenblock}{Research Questions}
\begin{enumerate}
\item Order $\alpha=?$
\item Asymptotic error constant $\beta=?$
\end{enumerate}
\begin{example}{An experimental study}
\begin{description}
\item [EA-I:] $(1+1)$ EA using onebit mutation and elitist selection
\item [EA-II:] $(1+1)$ EA using bitwise mutation and elitist selection
\item [ $f(x)$:] OneMax function
\item [$\alpha$:] set to 1
\end{description}
\begin{figure}[ht]
\begin{center}
\begin{tikzpicture}
\begin{axis}[domain=0:100, width=0.6\textwidth, height=0.1\textheight,
scaled ticks=false,
legend style={draw=none},
legend pos=outer north east,
xlabel={$t$},
ylabel={$E_t/E_{t-1}$},
%ymode=log,
]
\addplot[red ] table {Const-EAIf1n.dat};
\addplot [blue ] table {Const-EAIIf1n.dat};
\legend{EA-I, EA-II}
\end{axis}
\end{tikzpicture}
\end{center}
\caption{For EA-I and II, ${E_t}/{E_{t-1}}$ converge to some $\beta$ but stochastic disturbance exists on EA-II.}
\end{figure}
\end{example}
\end{greenblock}
\begin{greenblock}{Analysis Tool}
The analysis tool is Markov chain theory~\cite{he2003towards,he2016average}.
Label all populations by indexes $\{ 0, 1, \cdots, L\}$ where indexes are sorted according to the fitness value of populations from high to low:
\begin{align}
f_{\max} =f_0 > f_1 \ge \cdots \ge f_L = f_{\min},
\end{align}
where $f_i$ denotes the fitness of the best individual in the $i$-th population.
\begin{itemize}
\item $r_{i,j}$ denotes the transition probability of an EA from $j$ to $i$.
\item Matrix $\mathbf{R} $ denotes transition probabilities within the set $\{1, \cdots, L \}$.
\begin{align}
\mathbf{R}: = &\begin{pmatrix}
r_{1,1} & r_{1,2}& r_{1,3}& \cdots & r_{1,L-1} &r_{1,L} \\
r_{2,1} & r_{2,2} & r_{2,3}& \cdots & r_{2,L-1} &r_{2,L}\\
r_{3,1} & r_{3,2} & r_{3,3} & \cdots & r_{3,L-1} &r_{3,L}\\
\vdots & \vdots & \vdots & \vdots & \vdots \\
r_{L,1} & r_{L,2} & r_{L,3} &\cdots & r_{L,L-1} & r_{L,L} \\
\end{pmatrix}.
\end{align}
\item Vector
$
\mathbf{q}_0 \defeq (\Pr(1), \Pr(2), \cdots, \Pr(L) )^T
$ represent the probability distribution of the initial population over the set $\{1, \cdots, L \}$.
\end{itemize}
Suppose that EAs can be modelled by homogeneous Markov chains and are convergent (approximation error $E_t \to 0$ when $t \to +\infty$).
\end{greenblock}
\end{column}
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\begin{column}{0.45\paperwidth}
\begin{greenblock}{Main Theoretical Result}
\begin{enumerate}
\item In many cases, the order of convergence $\alpha=1$
\item Asymptotic error constant equals to the spectral radius: $\beta=\rho(\mathbf{R})$.
\end{enumerate}
\end{greenblock}
\begin{greenblock}{General EAs}
Under the particular initialization, that is, set $\mathbf{q}_0= \mathbf{v}/|\mathbf{v}|$ where $\mathbf{v}$ is an eigenvector corresponding to the eigenvalue $\rho(\mathbf{R})$~\cite{he2016average}.
\begin{theorem}{Theorem 1}
Let $\mathbf{R}$ be the transition submatrix with $\rho(\mathbf{R})<1$.
Under particular initialization , it holds for all $t\ge 1$,
\begin{align}
\frac{E_t}{E_{t-1}}
= \rho(\mathbf{R}).
\end{align}
That is $\alpha=1$ and $\beta= \rho(\mathbf{R})$.
\end{theorem}
\end{greenblock}
\begin{greenblock}{EAs with Primitive Transition Matrices}
Case 1: transition matrix $\mathbf{R}$ is primitive.
\begin{definition}{Primitive matrix}
A matrix $\mathbf{R}$ is called primitive if there exists a positive integer $m$ such that $\mathbf{R}^m > \mathbf{O}$.
This condition means that starting for any state $i$, an EA can reach any other state $j$ in $m$ generations.
\end{definition}
Under random initialization, that is, the initial population can be chosen to be any non-optimal state with a positive probability. Equivalently, $\mathbf{q}_0 > \mathbf{0}$.
\begin{theorem}{Theorem 2}
If $\mathbf{R} $ is primitive, then under random initialization, it holds
\begin{align}
\lim_{t \to +\infty}\frac{E_t}{E_{t-1}}
= \rho(\mathbf{R}).
\end{align}
That is $\alpha=1$ and $\beta= \rho(\mathbf{R})$.
\end{theorem}
\end{greenblock}
\begin{greenblock}{EAs with Reducible Transition Matrices}
Case 2: transition matrix $\mathbf{R}$ is reducible.
\begin{definition}{Definition}
$\mathbf{R}$ is reducible if it can be split as
\begin{equation}
\mathbf{R} = \begin{pmatrix}
\mathbf{R}_{11} & \mathbf{R}_{12} \cr
\mathbf{O} & \mathbf{R}_{22} \cr
\end{pmatrix}
\end{equation}
where $\mathbf{O}$ is a zero-value submatrix.
\end{definition}
Consider a special reducible transition matrix $\mathbf{R}$ that is an upper triangular matrix:
\begin{align}
\mathbf{R} = \begin{pmatrix}
r_{1,1} & r_{1,2}& r_{1,3}& \cdots & r_{1,L-1} &r_{1,L} \\
0& r_{2,2} & r_{2,3}& \cdots & r_{2,L-1} &r_{2,L}\\
0 & 0 & r_{3,3} & \cdots & r_{3,L-1} &r_{3,L}\\
\vdots & \vdots & \vdots & \vdots & \vdots \\
0& 0& 0&\cdots & 0& r_{L,L} \\
\end{pmatrix}.
\end{align}
\begin{theorem}{Theorem 3}
If $\mathbf{R} $ is upper triangular with unique diagonal entry $r_{i,i}$, then under random initialization, it holds
\begin{align}
\lim_{t \to +\infty}\frac{E_t}{E_{t-1}}
= \rho(\mathbf{R}).
\end{align}
That is $\alpha=1$ and $\beta= \rho(\mathbf{R})$.
\end{theorem}
\end{greenblock}
\begin{greenblock}{References}
\setbeamertemplate{bibliography item}{\insertbiblabel}
\begin{footnotesize}
\begin{thebibliography}{1}
\bibitem{burden2015numerical}
R.~Burden, J.~Faires, and A.~Burden, \emph{Numerical Analysis}.\hskip 1em plus
0.5em minus 0.4em\relax Cengage Learning, 2015.
\bibitem{gautschi2011numerical}
W.~Gautschi, \emph{Numerical Analysis}.\hskip 1em plus 0.5em minus 0.4em\relax
Birkh{\"a}user Boston, 2011.
\bibitem{he2003towards}
J.~He and X.~Yao, ``Towards an analytic framework for analysing the computation
time of evolutionary algorithms,'' \emph{Artificial Intelligence}, vol. 145,
no. 1-2, pp. 59--97, 2003.
\bibitem{he2016average}
J.~He and G.~Lin, ``Average convergence rate of evolutionary algorithms,''
\emph{IEEE Transactions on Evolutionary Computation}, vol.~20, no.~2, pp.
316--321, 2016.
\end{thebibliography}
\end{footnotesize}
\end{greenblock}
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\end{columns}
\contact{\color{white} Contact: Jun He,
Department of Computer Science, Aberystwyth University,
Aberystwyth SY23 3DB, UK. Email: jun.he@aber.ac.uk}
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\end{document}