Package ntheorem Error

Question

After compiling this script:

\documentclass[a4paper,10pt]{book}
\usepackage[frenchb]{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{geometry} 
\usepackage{amssymb,amsmath,mathtools, amsthm}
\usepackage{ntheorem} 
\newtheorem{theorem}{Théorème}
\begin{document}
Quelques dates importantes ont marquées  l’avancée spectaculaire de la recherche
sur les CW complexe.
\end{document}

I receive this error code:

! Package ntheorem Error: Theorem style plain already defined.

See the ntheorem package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.524 ...rfont ##1\ ##2\ (##3)\theorem@separator]}

! Package ntheorem Error: 
          Theorem style plain already defined.  
          See the ntheorem package documentation for explanation.   
          Type  H <return> 
          for immediate help.  ...                                              
          l.524 ...rfont ##1\ ##2\ (##3)\theorem@separator]}

I don't know where to turn, despite many changing.

Any help is welcome!