最小限のAtomフォーマット ref: http://qiita.com/items/2790

entry1.atom

<?xml version="1.0" encoding="UTF-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
  <id>tag:example.com,2012:1234</id>
  <title>sample entry</title>
  <updated>2012-01-01T00:00:00+09:00</updated>
  <author><name>author name</name></author>
  <content>コンテンツの中身そのもの</content>
</entry>

entry2.atom

<?xml version="1.0" encoding="UTF-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
  <id>tag:example.com,2012:1234</id>
  <title>sample entry</title>
  <updated>2012-01-01T00:00:00+09:00</updated>
  <author><name>author name</name></author>
  <link href="http://example.com/foo.html"/>
</entry>

feed.atom

<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <id>tag:example.com,2012:1234</id>
  <title>sample feed</title>
  <updated>2012-01-01T00:00:00+09:00</updated>
  <link rel="self" href="http://example.com/self.atom"/>
  <author><name>author name</name></author>
</feed>