I want to make the diagram along with the labels (citation) that looks like:

My working is as follows:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{arrows,matrix,positioning,shapes,arrows}
\usetikzlibrary{shapes.geometric, arrows, calc, intersections}
\newcommand{\tikznode}[2]{\relax
    \ifmmode%
    \tikz[remember picture,baseline=(#1.base),inner sep=0pt] \node (#1) {$#2$};
    \else
    \tikz[remember picture,baseline=(#1.base),inner sep=0pt] \node (#1) {#2};%
    \fi}

\begin{document}    
    \begin{figure}[!h]
        \centering
        \begin{tikzpicture}
        [
        squarednode/.style={%
            rectangle,
            draw=black!60,
            fill=white,
            very thick,
            minimum size=5mm,
            text centered,
            text width=3cm,
        }
        ]
        %Nodes
        \node[squarednode]      (maintopic)                              {Graph};
        \node[squarednode]      (uppersquare)       [above=of maintopic] {$FTTM$};
        \node[squarednode]      (rightsquare)       [right=2.5cm of maintopic] {Pascal Triangle};
        \node[squarednode]      (lowersquare)       [above=of rightsquare] {Fibonacci};
        
        %Lines
        \draw[<->] (uppersquare.south) -- node[anchor=east] {} (maintopic.north);
        \draw[<->] (maintopic.east) -- node[anchor=south] {} (rightsquare.west);
        \draw[<->] (rightsquare.north) -- node[anchor=west] {} (lowersquare.south);
        \draw[<->] (uppersquare.east) -- (lowersquare.west);
        \end{tikzpicture}
        \caption{Three mathematical concepts which are linked to $FTTM$.}
        \label{figure:link}
    \end{figure}
\end{document}

which produces:

Please help.