The "Sigma" entity is mapped to the wrong unicode point, see also http://www.w3.org/TR/html4/sgml/entities.html
<!-- there is no Sigmaf, and no U+03A2 character either -->
<!ENTITY Sigma CDATA "Σ" -- greek capital letter sigma,
U+03A3 ISOgrk3 -->
Moreover "apos" (ASCII code 39) is not recognized; it seems to be not in traditional HTML4, but XHTML certainly has it (qua XML standard).