Error - DOI Naming Authority [http:] Not Found

The DOI you requested --

http://doi.acm.org/10.1145/1104973.1104980

-- cannot be found in the Handle System.

Possible reasons for the error are:

• the DOI has not been created
• the DOI is cited incorrectly in your source
• the DOI does not resolve due to a system problem

If you believe you have requested a DOI that should be found, you may report this error by filling out the form below: