This is the official manual for the latest Org-mode release.
Special characters that are written in TeX-like syntax, such as
\Zeta, are supported by DocBook exporter. These
characters are rewritten to XML entities, like
Ζ, based on the list saved in variable
org-entities. As long as the generated DocBook file includes the
corresponding entities, these special characters are recognized.
You can customize variable
org-export-docbook-doctype to include the
entities you need. For example, you can set variable
org-export-docbook-doctype to the following value to recognize all
special characters included in XHTML entities:
"<!DOCTYPE article [ <!ENTITY % xhtml1-symbol PUBLIC \"-//W3C//ENTITIES Symbol for HTML//EN//XML\" \"http://www.w3.org/2003/entities/2007/xhtml1-symbol.ent\" > %xhtml1-symbol; ]> "