Although I've been using Latex for many years, I just understand the minimum to survive and don't have clue of where the problem can come from. So here are the loaded packages.
\documentclass[10pt,a4paper,titlepage]{book}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}% math environment, e.g. \begin{align}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}% \boldsymbol in math
\usepackage[amsmath,amsthm,thmmarks]{ntheorem}% add \theoremsymbol{\ensuremath{\clubsuit}} to environment
http://tex.stackexchange.com/questions/20575/attractive-boxed-equations
\usepackage{multirow}% multiple rows tabular cells; 2nd Proof Volume of Bn within "bigdelim; bigstrut" packages
\usepackage{blkarray}% label rows and colums of a matrix; \begin{blockarray} and \begin{block}
\usepackage{graphics} % \includegraphics{}
\usepackage[dvips]{graphicx}% provides options \includegraphics[scale=1]{file}
\usepackage{rotating} % rotate sthg (grosse matrice jacobienne)
\usepackage{MnSymbol} % only \lcirclearrowdown arrow in commutative diagram, or \downspoon
\usepackage[all]{xy} % writing commutative diagrams
\usepackage{enumitem} % change the margin in \begin{itemize} and alike [leftmargin=1in]
\usepackage{bbm} % unit matrix \mathbbm{}
\usepackage{marvosym} % plus de symbol, symbols généraux non spécifiques aux maths. Contradiction \Lightning, @
\usepackage{comment} % allows to put whole parts as comment and not % line by line
\usepackage[usenames,dvipsnames]{xcolor} % [get more than the predefined 8 colors]
\usepackage{soul} % for highlighting. hyphenation, letter spacing, underline etc.
\newcommand{\hlc}[2][Yellow]{\sethlcolor{#1} \hl{#2}}
\usepackage{marginnote} % align comments in marge with some environment, allow use in eq. env.
\usepackage{cancel} % slash for gamma matrices
\usepackage[normalem]{ulem} % to do fancy underlying, [doesn't change \emph] , also useful when linebreak
\usepackage{tcolorbox} % also loads pgf, etoolbox, verbatim, environ packages; support boxes over multiple pages
\tcbuselibrary{breakable} \tcbuselibrary{skins} \tcbset{enhanced}
\usepackage{makeidx} % write \index{}
\usepackage{fullpage} % extend broadth cf. also \usepackage[a4paper]{geometry}: set margins
\usepackage{tikz} % draw geometric figures, \begin{tikzpicture}; place after xcolor because conficts
\usepackage{titlesec} % \chapter command; option: no page break after chapter with titleclass "straight" statt top
\titleclass{\chapter}{top}
\titleformat{\chapter}[display]
{\normalfont\huge\bfseries}{}{0pt}{\Huge}
\titlespacing*{\chapter}{0pt}{30pt}{25pt}
\usepackage{hyperref} % comme pour les equations, utilise \label et \hyperref[key]{text}
\hypersetup{
colorlinks,
citecolor=black,
filecolor=black,
linkcolor=blue,
urlcolor=black
} % no red framebox...
\theoremstyle{definition} % option de \newtheorem
\theoremsymbol{\ensuremath{\heartsuit}}
\newtheorem{definition}{Definition}[section]
\theoremsymbol{\ensuremath{\clubsuit}}
\newtheorem{lem}{Lemma}[section]
\newtheorem{coro}{Corollary}[section]
\newtheorem{prop}{Property}[section]
\newtheorem{propos}{Proposition}[section]
\newtheorem{thm}{Theorem}[section]
\setlength{\ULdepth}{\maxdimen}
%\addtolength{\ULdepth}{2pt}
\newenvironment{rem}{\vspace{0.5\baselineskip} \par \noindent\begin{color}{BlueViolet}\begin{small}\textit{\uline{Remark \thesection} :} \refstepcounter{section} }{\end{small}\end{color} \newline}% \newenvironment{name}{before text} text {after}
\newenvironment{rems}{\vspace{0.5\baselineskip}\par\noindent\begin{color}{BlueViolet}\begin{small}\textit{\uline{Remarks \thesection} :} \refstepcounter{section} \begin{enumerate}}{\end{enumerate}\end{small}\end{color}}
I have seen how get the interior product/contraction symbol $\lrcorner$ which works perfectly in math.stackexchange, but in my latex file the symbol appears very small and under the line.
I've tried $\mathrel{\lrcorner}$ as seen in another question or ${\big \lrcorner}$ as in geometric algebra, wikipedia but it has no effect.
