Reply to: syntax for special charactersMay 21, 2015 at 8:54 pm
The DITA DTDs do not define any named character entities* (which is why you are being told the file is invalid as you are referencing an undefined entity). It is best to do as barbwire says and insert Unicode characters directly into the document.
*Note that this is not entirely true. The DITA DTDs define one named character entity but it is not recommended for use:
It was included in the original DTDs that IBM created and remains in all versions up to 1.3 but is marked as “deprecated” and will be removed in the 2.0 version of the DTDs.