Lining *and* oldstyle figures in maths

Question

Here is my problem: I would like, as in some old maths books, use oldstyle figures as coefficients in math formulae – say in display- and textstyle, but lining figures for indices and exponents – say in script- and scriptscriptstyle. I'm not sure this is strictly equivalent, though. I guess that the use of the \mathchoice command would be useful, but I really don't know how to use it.

Does anyone know how to achieve this automatically?

Added: I forgot to say that a solution should work with any math font. It happens that the proposed solution, which works perfectly well with default TeX math fonts doesn't work with MinionPro. Maybe a specific problem of MinionPro?

Answer 1

I wouldn't do it. Old books are not always examples of good typography.

\documentclass{article}

\makeatletter
\def\changedigit#1{%
  \begingroup\lccode`~=`#1\lowercase{\endgroup
    \edef~}{{\mathchoice
      {\noexpand\mathnormal{\mathchar\the\mathcode`#1}}
      {\noexpand\mathnormal{\mathchar\the\mathcode`#1}}
      {\mathchar\the\mathcode`#1}
      {\mathchar\the\mathcode`#1}
    }}
  \AtBeginDocument{\mathcode`#1=\string"8000 }
}
\@for\next:=1,2,3,4,5,6,7,8,9,0\do{\expandafter\changedigit\next}
\let\changedigit\@undefined
\makeatother

\begin{document}

$1234567890^{1234567890}$

$10x_0 + 11y_1$

\end{document}

You might want to add \scriptscriptstyle in the third argument to \mathchoice, which would give

or play with \DeclareMathSizes (see https://tex.stackexchange.com/a/58144/4427)

If you want to have all digits always oldstyle, change the code between \makeatletter and \makeatother into

\makeatletter
\@for\next:=1,2,3,4,5,6,7,8,9,0\do{%
  \begingroup\edef\x{%
    \mathcode`\next=\noexpand\number\numexpr\mathcode`\next-"7000+"100\relax}%
    \edef\x{\endgroup\x}\x
}
\makeatother

Of course, this last code can be substituted with

\DeclareMathSymbol{0}{\mathord}{letters}{`0}

and similar lines for the other digits.