I've been trying to typeset as an operator the symbol ⫇ (\subset on top a \sim), but the following solutions have not been helpful
\limits and \underset puts the tilde (or \sim) too separated from \subset.\undertilde looks ugly.Is any other option to obtain this symbol?
Adapting egreg's solution:
\documentclass{article} \usepackage{graphicx} \newcommand\subsetsim{\mathrel{% \ooalign{\raise0.2ex\hbox{$\subset$}\cr\hidewidth\raise-0.8ex\hbox{\scalebox{0.9}{$\sim$}}\hidewidth\cr}}} \begin{document} \[ A \subsetsim B \] \end{document} 
Adapt the parameters to your likings.
Here's a straight-forward way using \substack from amsmath:
\documentclass{article} \usepackage{amsmath}% http://ctan.org/pkg/amsmath \newcommand\subsetsim{\mathrel{\substack{ \textstyle\subset\\[-0.2ex]\textstyle\sim}}} \begin{document} \[ f(x) \subset g(x) \subseteq h(x) \subsetsim i(x) \sim j(x) \] \end{document} The definition could be updated to accommodate usage in sub-/superscripts as well.
A you have noticed the symbol is in Unicode as ⫇ (U+2AC7) so if you are using xetex or luatex and unicode-math it would just work. It's also in the STIX fonts of which there is a beta release now with support for classic TeX: http://www.stixfonts.org/
\sqsubset over \sim? The constructed symbol here has the same height as \subseteq.
\documentclass{article} \usepackage{amsmath} \usepackage{graphicx} \makeatletter \newcommand{\subsim}{\mathrel{\mathpalette\su@sim\subset}} \newcommand{\supsim}{\mathrel{\mathpalette\su@sim\supset}} \newcommand{\su@sim}[2]{% \begingroup \sbox\tw@{$\m@th#1\mkern-1mu\subseteq\mkern-1mu$}% \raisebox{\dimeval{\ht\tw@-\height}}{% \vbox{\m@th\offinterlineskip \sbox\z@{$#1\mkern1mu$}% for the vertical spacings \ialign{% \hfil##\hfil\cr $#1#2$\cr \noalign{\vskip \wd\z@} \ifx#2\supset \reflectbox{\resizebox{\wd\tw@}{!}{$#1\sim$}}% \else \resizebox{\wd\tw@}{!}{$#1\sim$}% \fi\cr \noalign{\vskip -2\wd\z@} }% }% }% \endgroup } \makeatother \begin{document} \[ A\subsim B\qquad A\supsim B \] \begin{center} $A\subsim B\subseteq C \qquad A\supsim B\supseteq C$ \\ $\scriptstyle A\subsim B\qquad A\supsim B$ \\ $\scriptscriptstyle A\subsim B\qquad A\supsim B$ \\ \fboxsep=0pt \fboxrule=0.1pt \fbox{$\subsim$} \fbox{$\subseteq$} \end{center} \end{document} 
If the reflection of \sim is not desired, replace
\ifx#2\supset \reflectbox{\resizebox{\wd\tw@}{!}{$#1\sim$}}% \else \resizebox{\wd\tw@}{!}{$#1\sim$}% \fi\cr with just
\resizebox{\wd\tw@}{!}{$#1\sim$}\cr 
\supsim and \subseteq to show that they're both tight. \fbox, of course. \fboxsep=0pt \fboxrule=0.1pt \fbox{$\subsim$} \fbox{$\subseteq$}. Excuse me. The following example constructs the symbol ⫇ by putting \sim below \subset. The \sim is set in a smaller font, if the current math style is \displaystyle or \textstyle, then \scriptstyle is used for \sim, if the current math style is \scriptstyle or \scriptscriptstyle then \scriptscriptstyle is used for \sim. Since in the latter case the font is not smaller, this solution works for \displaystyle, \textstyle, and \scriptstyle.
\documentclass{article} \makeatletter \newcommand*{\subsetsim}{% \mathrel{% \mathchoice {\@subsetsim\displaystyle\scriptstyle}% {\@subsetsim\textstyle\scriptstyle}% {\@subsetsim\scriptstyle\scriptscriptstyle}% {\@subsetsim\scriptscriptstyle\scriptscriptstyle}% }% } \newcommand*{\@subsetsim}[2]{% \sbox0{$\m@th#1\subset$}% \sbox2{$\m@th#2\sim$}% \sbox4{$\m@th#2$}% \raisebox{.5\dimexpr\ht2-\ht4\relax}{% \oalign{\copy0 \cr\hidewidth\copy2\hidewidth}% }% } \begin{document} \[ a \subset a \sim a \subsetsim a - a_{b \subsetsim b_{c \subsetsim c}} \] \end{document} Remarks:
\sim is a little too large because the expression is set in \scriptscriptstyle, see above.Used only the \subsim symbol from stix package.
\documentclass[a4paper,12pt]{article} \usepackage{amsmath} \DeclareFontEncoding{LS1}{}{} \DeclareFontSubstitution{LS1}{stix}{m}{n} \DeclareSymbolFont{symbols3} {LS1}{stixbb} {m} {n} \DeclareMathSymbol{\subsim}{\mathrel}{symbols3}{"5F} \begin{document} $A\subsim B\subsim C \subsim D\subseteq E$ \end{document} 