How to write Rudin's symbol for absolute continuity of measures?

In Rudin's Real and Complex Analysis, there is a symbol for stating measure $\lambda$ is absolutely continuous with respect to measure $\mu$. The symbol is like $\lambda\ll\mu$, but not exactly.

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If one doesn't want to load the whole lot of mathabx, that changes the shape of many symbols, it's possible to load only the one needed, see Importing a single symbol font from a different font.

The definition would be

\DeclareMathSymbol{\Lt}{3}{matha}{"CE}


using the unicode-math name or, if the alternative way by pisymbol is preferred,

\newcommand{\Lt}{\Pimathsymbol[\mathrel]{matha}{"CE}}


The complete series of declarations in the preamble should be

\DeclareFontFamily{U}{matha}{\hyphenchar\font45}
\DeclareFontShape{U}{matha}{m}{n}{
<-6> matha5 <6-7> matha6 <7-8> matha7
<8-9> matha8 <9-10> matha9
<10-12> matha10 <12-> matha12
}{}
% \DeclareFontShape{U}{matha}{m}{n}{
%   <5> <6> <7> <8> <9> <10> gen * matha
%   <10.95> matha10 <12> <14.4> <17.28>
%   <20.74> <24.88> matha12
%   }{}

\DeclareSymbolFont{matha}{U}{matha}{m}{n}
\DeclareMathSymbol{\Lt}{3}{matha}{"CE}


If your TeX distribution is fairly recent you can leave out the commented lines (starting with %); otherwise, uncomment them and delete the similar ones above. It has to do with the presence of the Type1 (scalable) versions of the mathabx fonts.

With the alternative method that doesn't reserve a math alphabet, the code to write in the preamble is

\usepackage{pifont}
\DeclareFontFamily{U}{matha}{\hyphenchar\font45}
\DeclareFontShape{U}{matha}{m}{n}{
<-6> matha5 <6-7> matha6 <7-8> matha7
<8-9> matha8 <9-10> matha9
<10-12> matha10 <12-> matha12
}{}
\makeatletter
\newcommand\Pimathsymbol[3][\mathord]{%
#1{\@Pimathsymbol{#2}{#3}}}
\def\@Pimathsymbol#1#2{\mathchoice
{\@Pim@thsymbol{#1}{#2}\tf@size}
{\@Pim@thsymbol{#1}{#2}\tf@size}
{\@Pim@thsymbol{#1}{#2}\sf@size}
{\@Pim@thsymbol{#1}{#2}\ssf@size}}
\def\@Pim@thsymbol#1#2#3{%
\mbox{\fontsize{#3}{#3}\Pisymbol{#1}{#2}}}
\makeatother
\newcommand{\Lt}{\Pimathsymbol[\mathrel]{matha}{"CE}}


For the \DeclareFontShape command it's the same as before.

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thank you for your answer. I do want to be able to import the symbols needed only. I am having some trouble here to do that. I tried with \DeclareFontFamily{U}{matha}{\hyphenchar\font45} \DeclareFontShape{U}{matha}{m}{n}{ <5> <6> <7> <8> <9> <10> gen * matha <10.95> matha10 <12> <14.4> <17.28> <20.74> <24.88> matha12 }{} \DeclareMathSymbol{\Lt}{3}{matha}{"CE} , but I am getting an error Symbol font matha' is not defined. – Qiang Li Jun 8 '11 at 23:51
@Qiang Li: you missed the \DeclareSymbolFont{matha}{U}{matha}{m}{n} line. I've added the complete code. – egreg Jun 9 '11 at 0:06
+1 and thank you. Now it is working! So I assume the first approach is to use the alias in the unicode-math. How to use the symbol directly, as Caramdir said in his/her post? – Qiang Li Jun 9 '11 at 0:36

Have a look at “How to look up a math symbol?” for ideas how you can easily find a particular symbol.

The symbols list has \lll from the mathabx package (note that you need to load mathabx after amsmath, because amsmath defines \lll as ⋘.

Unicode defines the symbols as ⪡ U+2AA1 DOUBLE NESTED LESS-THAN. If you use unicode-math, you can use the symbol directly or use the alias \Lt.

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@egreg: thank you for your answer. I do want to be able to import the symbols needed only. I am having some trouble here to do that. I tried with \DeclareFontFamily{U}{matha}{\hyphenchar\font45} \DeclareFontShape{U}{matha}{m}{n}{ <5> <6> <7> <8> <9> <10> gen * matha <10.95> matha10 <12> <14.4> <17.28> <20.74> <24.88> matha12 }{} \DeclareMathSymbol{\Lt}{3}{matha}{"CE} , but I am getting an error Symbol font matha' is not defined. – Qiang Li Jun 8 '11 at 20:38
@Qiang: Your comment is on the wrong answer. – Caramdir Jun 8 '11 at 23:40