MathML ApplyFunction Entity - where does it come from?

I'm using System.Xml.Linq to parse MathML 2.0 via its associated DTD. Everything is fine except that Maple produces the ⁡ element which does not appear to be a DTD. Where is this element defined? I tried googling, but to no a开发者_StackOverflow中文版vail.

⁡ is an entity that's treated as an operator (i.e., an mo element) in MathML. Its a valid Unicode character, with codepoint x2061: http://www.fileformat.info/info/unicode/char/2061/index.htm

ApplyFunction is normally used in order to prevent ambiguity, rather than as a required operator. For example, this code block

<mi>sin</mi><mo>(</mo><mi>x</mi><mo>)</mo>

is just as valid as this code block

<mi>sin</mi><mo>&ApplyFunction;</mo><mo>(</mo><mi>x</mi><mo>)</mo>

and really there's no ambiguity in either case, but for some functions there may be.

⁡ should appear as an entity declaration in any MathML DTD.