Dear all,

I've got a quick question about the use of subscripts in Content MathML which I hope someone can shed some light on.

(a) Background:

Basically, this problem arose with the study of sums; it is often useful to describe a summation using a single algebraic letter with a subscript denoting the exact term. For example:

n

Sum ( a_i ) = a_1 + a_2 + a_3 + ... + a_n

i = 1

where the a_i notation means "a subscript i". There are lots of instances where this notation is used, and I'm sure everyone has come across it before; one elementary example is in series (for example, see http://mathworld.wolfram.com/Sum.html).

(b) Problem

The MathML spec. suggests than any need for subscripts within Content markup should be made using Presentation embedded within Content. For example, one way to describe a_i would be:

<ci><msub><mi>a</mi><mi>i</mi></msub></ci>

The problem with this is that the identifiers 'a' and 'i' are presentation-only, and hence could not be operated on by our algebraic processor engine (which uses a Content-only DOM).

(c) Solution

What we would like to do is the following:

<ci><msub><ci>a</ci><ci>i</ci></msub></ci>

Here we're mixing the two types of mark-up by embedding Content within Presentation within Content. However, when the processing engine evaluates the sum, it can easily replace all occurances of <ci>i</ci> with <cn>1</cn>, <cn>2</cn> etc... which would give the required terms (a_1, a_2 etc...) in the sum.

My question is really whether this is VALID mark-up? I believe it probably is [DTD definition p.301 MathML 2.0 spec., entities %PresExpression and %ContInPres] but I want to make sure with the experts that I am correct before proceeding.

Another question: in order to render mixed mark-up in a user-agent, would an XSLT transform need to be applied on the content elements?

Thanks in advance,

Charles Lyons.

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