Is xmlns="" is a valid xml namespace?

Question

Is an empty xml namespace is a valid value. If yes what does it means?

I have XML which looks like following one but I'm not sure what namespace Field1 and Field2 belong to.

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"&gt;
  <soapenv:Header />
  <soap:Body>
    <Root xmlns="uri">
      <Field1 xmlns="">147079737</Field1>
      <Field2 xmlns="">POL</Field2>
    </Root>
  </soap:Body>
</soapenv:Envelope>

Answer 1

Yes, it is valid. Section 6.2 in the Namespaces in XML 1.0 Recommendation specifically says:

The attribute value in a default namespace declaration MAY be empty. This has the same effect, within the scope of the declaration, of there being no default namespace.