# Undefined control sequence error while using \tagaddtext

$$\begin{split} H(m_X) &= \sqrt{\frac{8\pi G}{3} \rho(x=1)}\\ %removed big before rho from here from here. \rho(x=1) &= \epsilon / c^2 &= \frac{g_*\pi^2}{30}\frac{K_b^4T_m^4}{\hbar^3c^5}\\ \tagaddtext{\Big(T_m = T(x=1)\Big)}\\ \end{split}$$


So here is my code. Iam using the packages delimset and amsmath.

My error is.

! Undefined control sequence.
{\Big (T_m = T(x=1)\Big )}...
l.223 \end{split}


The thing that I found was that by commenting on the \tagaddtext. The file runs perfectly. So I would like to know whats causing the issue. Thanks in advance and I am a beginner in LaTeX. Started like around 5 days back.

You seem to have multiple mixed constructions in that section.

Thus it is difficult to guess your desired output.

You have wrapped the equation matter in {split} but I don't see any split defined (or any need for one)

You have included alignment markers &= suggesting you simply need the equations in a basic {align}

The problem with \tagaddtext is that it is an undefined command that comes from an answer by Werner to Numbered equations with additional text part 2 so needs to be defined the same way in the preamble and as text when in a math environment it needs as a minimum $...$. Adding all the above comments together I get this as a working solution with the given snippet.

\documentclass{article}
\usepackage{delimset,amsmath}% http://ctan.org/pkg/amsmath

\makeatletter
%\begin{align} %\begin{split} H(m_X) &= \sqrt{\frac{8\pi G}{3} \rho(x=1)}\\ \rho(x=1) &= \epsilon / c^2 \\ &= \frac{g_*\pi^2}{30}\frac{K_b^4T_m^4}{\hbar^3c^5} \tagaddtext{\Bigl(T_m = T(x=1)\Bigr)} %\end{split} \end{align} %
`