[Tex/LaTex] Double subscript caused error during compilation

Question

I have in code:

  $\varphi(x) = \sum_{k=1}^ra_k\mathds{1}_{B_k}(x) $

but pdflatex commmand generates often:

  ? 
  ! Undefined control sequence.
  l.1354 ...rc \xi)(\omega) = \sum_{k=1}^ra_k\mathds
                                              {1}_{B_k}(\xi(\omega))

I don't know how to repair this error. It often displays because I use \mathds{1} for indicator function. How to fix it ?

Edit:

\mathds{} comes from \usepackage{dsfont}

Edit: Below is my class

\documentclass[12pt, a4paper]{article}

\usepackage[top=3.5cm, bottom=3.5cm, outer=1.5cm, inner=2cm, marginparwidth=2.5cm, marginparsep=2cm]{geometry}
\usepackage[polish]{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[pdftex, linkbordercolor={0 0.9 1}]{hyperref}
\usepackage{amsthm,amsmath,amsfonts,amssymb,mathtools}
\usepackage{enumerate}
\usepackage{graphicx}
\usepackage{bbm}
\usepackage{fancyhdr}
\usepackage{marginnote}


\pagestyle{fancy}
 \fancyhf{}
\fancyfoot[R]{\textbf{\thepage}}
\fancyhead[L]{\small\sffamily \nouppercase{\leftmark}}


 % indicator function
 \usepackage{dsfont} 
 \DeclareMathOperator{\1}{\mathds{1}}

 \begin{document}
    $\varphi(x) = \sum_{k=1}^ra_k\mathds{1}_{B_k}(x) $
 \end{document}

Answer 1

You can install the LaTeX package dsfont.sty with the means of your TeX distribution:

• TeX Live package name: doublestroke
• MiKTeX package name: dstroke