I would like to write above equation in my document. But I am getting Missing { error. Please help me with this code.

$$\vec{X}(t+1)={\vec{D}^'\.e^{bl}\.\cos(2\pil)+\vec{X}^*(t)}$$


So, you have several problems with your code:

• you use ^' while the simple ' is sufficient
• you use \. instead of \cdot
• you use the mathematical shift \$ inside of an equation (which is unnecessary)
• you don't provide a minimal example

Here is an example

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}
\begin{document}
$$\vec{X}(t+1)=\vec{D'}\cdot e^{bl}\cdot \cos(2\pi l)+\vec{X^{*}}(t)$$
\end{document}


which produces

If you want longer arrows you could use \renewcommand{\vec}{\overrightarrow}

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}
\renewcommand{\vec}{\overrightarrow}
\begin{document}
$$\vec{X}(t+1)=\vec{D'}\cdot e^{bl}\cdot \cos(2\pi l)+\vec{X^{*}}(t)$$
\end{document}


I would suggest \vv from the esvect package. It has 8 arrow tips ( from a to h, which do not touch capital letters, and are extensible. Here are some examples:

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}
\usepackage[b]{esvect}
\renewcommand{\vec}{\overrightarrow}
\begin{document}

$$\vec{X}(t+1)=\vec{D'}\cdot e^{bl}\cdot \cos(2\pi l)+\vec{X^{*}}(t)$$
$$\vv{X}(t+1)=\vv{D'}\cdot e^{bl}\cdot \cos(2\pi l)+\vv{X^{*}}(t)$$

\end{document}


Option b:

Option d (default):

Option f: