## 4 Result of the Systematic Literature Review
### Research Strategy Analysis
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Reference** & **SR Proposed Method** & **Number of Targets(s)** & **Targets/Agents Ratio** & **Mobility of Targets** & **Environment Complexity** & **Verification Method** & **Inspiration Phenomenon** \\ \hline
[41] & Kinematic constrain target search strategy & Single & \(<\)1 & Static & Empty space & Simulation & Flocking behavior of birds \\
[22] & Group explosion strategy & Multiple & \(>\)1 & Static & Empty space & Simulation & Explosion phenomenon in nature and flocking behavior of birds \\
[34] & Three restriction-handling strategies & Multiple & \(<\)1 & Static & Cluttered & Simulation &
[35] & Decoy-avoiding strategies & Multiple & \(<\)1 & Static & Cluttered & Simulation & \
\begin{tabular}{c} **Verification** \\ **Method** \\ \end{tabular} & **Inspiration Phenomenon** \\ \hline Modified bacterial & & & & & & & \\ foraging optimization & Multiple & \(<\)1 & Static & Cluttered & Simulation &
\begin{tabular}{c} **Foreaging behavior of birds** \\ **backera (_Escherichia coli_)** \\ \end{tabular} \\
[37] & foraging behavior & Multiple & \(<\)1 & Dynamic & Empty space & Simulation &
[38] & Self-organized target search and trapping strategy & Multiple & \(<\)1 & Dynamic & Empty space & Simulation & \
\begin{tabular}{c} **Foreaging behavior of** \\ **backera (_Escherichia coli_)** \\ \end{tabular} \\ Integrated strategy based & & & & & & & \\ on a modified particle & & & & & & & \\ swarm optimization (PSO) & Multiple & \(<\)1 & Static & Empty space & Simulation & Flocking behavior of birds \\ algorithm and a grouping & & & & & & & \\ strategy & & & & & & & \\ comparative between & & & & & & & \\ PSO, ant colony & & & & & & & \\ optimization (ACO) and genetic algorithm (GA) in & Single & \(<\)1 & Static & Cluttered & Simulation &
\begin{tabular}{c} **Flocking behavior of birds** \\ **and natural selection by** \\ **backera (_Escherichia coli_)** \\ \end{tabular} \\
[40] & Triangle formation search & Multiple & \(<\)1 & Static & Empty space & Simulation &
\begin{tabular}{c} Simulation \\ **and real robot** \\ \end{tabular} \\
[53] & A pheromone underwater robot monitoring strategy & Multiple & \(<\)1 & Static & Empty space &
\begin{tabular}{c} **Simulation** \\ **and real robot** \\ \end{tabular} \\ Repulsion-based robotic & & & & & & & \\ Darwinian particle swarm optimization (RDPSO) & & & & & & \\ (54) & & & & & & \\ (55) & & & & & & \\ (56) & & & & & & \\ cooperation strategy & & & & & & \\ A distributed strategy for & & & & & & \\ multi-target search in an & & & & & \\ unknown environment & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A summary of the most recent papers for target search problem with environment constraint. Analysis of the review was carried out, and the result verified that 64% (16/25) of the proposed methods used the SI approach in their SR strategies. Particle swarm optimization (PSO) was the most preferred SI algorithm, with 9/16 research papers having implemented it. PSO was originally suitable for solving global optimization problems, and target search problems had a similar characteristic of global optimization problems especially in multi-target search problems. Six of the papers chose the behavior-based approach in their SR strategies and the remaining three papers implemented the random walk method in their SR strategies to tackle the target search problem. Data from Table 2 also shows that 56% (14/25) of the papers set their experiments with multiple targets over single targets, and in 96% (24/25) of papers, the search agents were more than an available target. In terms of the target's mobility, 84% (21/25) of the papers set their targets in static conditions. These results show that the researchers focused on emerging swarm behaviors and interactions between the search agents during the target search problem. Review analysis also revealed that 64% (16/25) of research papers set the search environment space without any obstacles (empty space).