On 17/01/2017 12:00 a.m., Frank Mittelbach wrote:
> Am 16.01.2017 um 11:41 schrieb David Carlisle:
>>
>> Any apparent correspondence between inner and outer math and
>> displaystyle and textstyle is more or less accidental.
>
>
> \ifinner is described in the TeXbook simply as
>
> True if \TeX\ is in internal vertical mode, or restricted
> horizontal mode, or (nondisplay) math mode (see Chapter~13).
>
> which is unfortunately an oversimplification as the "style" can be
> explicitly set via \displaystyle and \textstyle and that is not
> reflected by the \ifinner test.
>
> Most math environment do exactly this, ie typesetting the formulas using
> $...$ = \ifinner=true but changing the style to \displaystyle when
> needed (with still \ifinner=true) which makes \ifinner pretty useless
> inside math
>
> As Joseph said, that is really only reflected when you look at \mathchoice
>
Thank you to all respondents who have made it clear that
"Math modes can be tested for: \ifmmode is true in display and
nondisplay math mode, and \ifinner is true in nondisplay mode, but not
in display mode"
(Eijkhout, "TeX by Topic", Section 23.1) is by no means generally true.
(Although I suspect that in the *particular* circumstances that I was
using the \ifinner test it does the job.)
Andrew

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