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I wonder how to typeset the restriction of a function to some subdomain, a la $f|_A$ or $f\restriction_A$ , but with the vertical bar being as big as possible.

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It seems that one can just use \big| or \Big|. But is there something which chooses the optimal size (or can there not be such a thing)? –  Michael Jul 4 '11 at 7:50
And welcome to TeX.SE! –  ℝaphink Jul 4 '11 at 8:33
For \restriction, \usepackage{amssymb}. –  Evgeni Sergeev Nov 22 '13 at 6:18
@EvgeniSergeev Your answer is the best - why don't you make it one? –  Ari Brodsky Sep 9 at 6:47
@AriBrodsky It doesn't actually answer the OP's question (though it might be useful for folks like me who came to the page looking to answer the question restricted to its title :) –  Evgeni Sergeev Sep 10 at 1:56

3 Answers 3

If you want the vertical bar to be at least \big, the following doesn't have the spacing issues of Ulrich's solution:

\newcommand\restr[2]{{% we make the whole thing an ordinary symbol
  \left.\kern-\nulldelimiterspace % automatically resize the bar with \right
  #1 % the function
  \vphantom{\big|} % pretend it's a little taller at normal size
  \right|_{#2} % this is the delimiter

If you don't need the extra height for normal size symbols, comment the \vphantom line.

Use it as \restr{f}{A}.

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Does $\left.f\right|_A$ provide what you are looking for?

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How about \newcommand\restr[2]{\ensuremath{\left.#1\right|_{#2}}}, which can be used like this: $\restr{f}{A}$?

(Warning, haven't tested...)

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