# Making arrows between a TikZ matrix the same size

Here's a MWE of what I'm dealing with:

\begin{tikzpicture}
\matrix (m) [matrix of math nodes,
row sep=2mm,]
{
&\left(l_{P,P}/v_{[2]P}\right) & = & (P) & + & (P) & - & (2P) & - &
(\mathcal{O})     \\
&\left(l_{[2]P,P}/v_{[3]P}\right) & = & (P) & + & ([2]P) & - & ([3]P) & -
&(\mathcal{O})         \\
&\left(l_{[3]P,P}/v_{[4]P}\right) & = & (P) & + & ([3]P) & - & ([4]P) & - & (\mathcal{O})         \\
&\vdots \\
&\left(l_{[r-3]P,P}/v_{[r-2]P}\right) & = & (P) & + & ([r-3]P) & - & ([r-2]P) & - &(\mathcal{O})         \\
+ & \left(l_{[r-2]P,P}\right) & = & (P) & + & ([r-2]P) & + & (P) & - & 3(\mathcal{O})\\
\hline \\
& (f_{r,P}) & = & r(P) & & & & & - & r(\mathcal{O}) \\
};
\draw[thin, <->] (m-1-8) -- (m-2-6);
\draw[thin, <->] (m-2-8) -- (m-3-6);
\draw[thin, <->] (m-5-8) -- (m-6-6);
\end{tikzpicture}


This yields

Is there a suggested way to make the arrows look better? I think that just having them be the same size would be best, but if there are other ideas I'd be open to that?

Here is a proposal.

\documentclass[tikz, margin=3mm]{standalone}
\usetikzlibrary{matrix}

\begin{document}
\begin{tikzpicture}
\matrix (m) [matrix of math nodes,
row sep=2mm,]
{
&\left(l_{P,P}/v_{[2]P}\right) & = & (P) & + & (P) & - & (2P) & - &
(\mathcal{O})     \\
&\left(l_{[2]P,P}/v_{[3]P}\right) & = & (P) & + & ([2]P) & - & ([3]P) & -
&(\mathcal{O})         \\
&\left(l_{[3]P,P}/v_{[4]P}\right) & = & (P) & + & ([3]P) & - & ([4]P) & - & (\mathcal{O})         \\
&\vdots \\
&\left(l_{[r-3]P,P}/v_{[r-2]P}\right) & = & (P) & + & ([r-3]P) & - & ([r-2]P) & - &(\mathcal{O})         \\
+ & \left(l_{[r-2]P,P}\right) & = & (P) & + & ([r-2]P) & + & (P) & - & 3(\mathcal{O})\\
\hline \\
& (f_{r,P}) & = & r(P) & & & & & - & r(\mathcal{O}) \\
};
\draw[thin, <->] (m-1-8) to[out=-120,in=60] (m-2-6);
\draw[thin, <->] (m-2-8) to[out=-120,in=60] (m-3-6);
\draw[thin, <->] (m-5-8) to[out=-120,in=60] (m-6-6);
\end{tikzpicture}
\end{document}

• This is nice. Thank you! So that I can know how this works for next time, can you explain what the to and out and in mean? – Alex Mar 31 '18 at 13:59
• @Alex \draw (A) to[in=x,out=y] (B); just means "draw from A to B with the angle at A being x and the angle at B being y. TikZ will then make a smooth curve respecting these boundary conditions. – user121799 Mar 31 '18 at 14:05