How can we write this big one in latex?

Question

I want to write this 1 in latex, could anyone help me, please?

Answer 1

My welcome to TeX.SE. This is a standard question. The next time use this tool to search your symbol: http://detexify.kirelabs.org/classify.html

\documentclass{article}
\usepackage{dsfont}
\usepackage{amsmath,amssymb}
\begin{document}

$f \times \mathds{1}$
\end{document}

If you do not like the symbol 1 in the image, there is another option with LaTeX. Use bbold package as an example and image that I have included.

\documentclass{article}
\usepackage{bbold}
\usepackage{amsmath,amssymb}
\begin{document}

$f \times \mathbb{1}$
\end{document}