What is this non-detexifiable Math symbol K?

Question

I could not detexify this:

How can you get it in LaTeX?

Answer 1

A blackboard bold letter, often used for number sets. Package amssymb provides \mathbb:

\documentclass{article}
\usepackage{amssymb}
\begin{document}
\[ \mathbb{ABCDEFGHIJKLMNOPQRSTUVWXYZ} \]
\end{document}