# “Undefined control sequence” error message

While compiling, an error message "Undefined control sequence" for the fourth line below (starting with 1958*e.....). Any idea about the cause?

$$S = \left\{ \begin{array}{rcl} 1280*Q^{0.46}*(1.43-0.26*\log{A}) & \mbox{if} & Q<2 in \\ 1958*e^{-0.055}*Q*(1.43-0.26*\logA) & \mbox{if} & Q\geq2 in \end{array}\right.$$

• Welcome to TeX.SE! The culprit is \logA. Replace it by \log A, i.e. add a space. But you really may want to have a look at some intro, and use e.g. \begin{cases} ... \end{cases} and \text instead of \mbox and so on. – user121799 Apr 22 '18 at 1:55
• the error message which you do not show highlights exactly which command is undefined – David Carlisle Apr 22 '18 at 5:28
• You have \log{A} and \logA: both should be \log A (no braces and a space). – egreg Apr 22 '18 at 8:48

There is a big difference between

\log{A}


(line 3 of your code) and

\logA


(line 4 of your code), but they're both wrong and it should be \log A in both cases.

Actually, \log{A} and \log A produce the same output, but it wouldn't be the same for \log{(A+B) and \log (A+B) (check it).

You should use amsmath and input your equation as

$$S = \begin{cases} 1280Q^{0.46}(1.43-0.26\log A) & \text{if Q<2\,\mathrm{in}} \\ 1958e^{-0.055}Q(1.43-0.26\log A) & \text{if Q\geq 2\,\mathrm{in}} \end{cases}$$


(in mathematics, multiplication is very rarely denoted by an asterisk).

Full code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$$S = \begin{cases} 1280Q^{0.46}(1.43-0.26\log A) & \text{if Q<2\,\mathrm{in}} \\ 1958e^{-0.055}Q(1.43-0.26\log A) & \text{if Q\geq 2\,\mathrm{in}} \end{cases}$$

\end{document}


• It is corrected as you proposed Mr egreg, and now it's good. Thank's for help. – Momkin Apr 22 '18 at 12:35