mjhjmtu

This was inspired by How to have nice-looking symbols within an itemize environment? and Sigur's comment which remind me about the display style of math formulas and the fractions.

I know that, in the amsmath package there is already dfrac for us to have display-style fractions (as well as tfrac for in-line fraction).

Then why don't dsum, dprod, dint, etc. exist?

If they are not to exist, then why does dfrac exist?

Actually, we can define, for example, dint as

DeclareMathOperatordintdisplaystyleint

but I want to know the reason why there are no dint, etc. Or can you tell me the packages in which these d- are in?

The command dfrac exists for rendering multistory fractions, say

[
fracdfracab-dfraccd{dfracab+dfraccd
]

and is not generally intended for usage in an inline formula. It is not defined using the simplistic format displaystylefrac, but rather

% amsmath.sty, line 214:
newcommanddfracgenfrac0

where genfrac is

% amsmath.sty, line 221:
DeclareRobustCommandgenfrac[4]%
 def@tempa#1#2%
 edef@tempb@nx@genfrac@mathstyle#4%
 csname @@ifx @#3@overelse abovefi
 ifx@tempa@empty else withdelimsfiendcsname
 @tempb#1#2#3

% amsmath.sty, line 289:
def@genfrac#1#2#3#4#5#1begingroup#4endgroup#2#3relax#5
def@mathstyle#1%
 ifx@empty#1@emptyrelax
 elseifcase#1displaystyle % case 0
 ortextstyleorscriptstyleelsescriptscriptstylefifi

What happens with dfracab? By definition this becomes genfrac0ab, so

def@tempa%
 edef@tempb@nx@genfrac@mathstyle0%
 csname @@ifx @@overelse abovefi
 ifx@tempa@empty else withdelimsfiendcsname
 @tempbab

By definition of @mathstyle, @tempb is defined to be @genfracdisplaystyle@@over, so we're left with

@genfracdisplaystyle@@overab

which in turn becomes

displaystylebegingroup aendgroup@@overrelax b

and @@over is the primitive over. Note the braces around the whole construction. You may enjoy chasing the expansion of frac, binom and dbinom.

With int one cannot do displaystyleint, because this wouldn't confine the scope of displaystyle, nor displaystyleint, because this would not place correctly the limits. Indeed, the test file

documentclassarticle
usepackageamsmath

DeclareMathOperatordintdisplaystyleint
newcommandddintdisplaystyleint

begindocument

$dint_0^1fracx2,dx$

$ddint_0^1fracx2,dx$

enddocument

produces wrong output in either case:

Can one do something about this? Yes, but I don't think it's worth the pain. Anyway, here it is:

documentclassarticle
usepackageamsmath
usepackagexparse

NewDocumentCommanddinttlimits e_^%
 mathop
 displaystyleint
 IfBooleanT#1limits
 IfValueT#2_#2
 IfValueT#3^#3
 %


begindocument

$dint_0^1fracx2,dx$
$dintlimits_0^1fracx2,dx$
$dintfracx2,dx$

enddocument