  We study a question answering problem on a social network, where a requester
is seeking an answer from the agents on the network. The goal is to design
reward mechanisms to incentivize the agents to propagate the requester's query
to their neighbours if they don't have the answer. Existing mechanisms are
vulnerable to Sybil-attacks, i.e., an agent may get more reward by creating
fake identities. Hence, we combat this problem by first proving some
impossibility results to resolve Sybil-attacks and then characterizing a class
of mechanisms which satisfy Sybil-proofness (prevents Sybil-attacks) as well as
other desirable properties. Except for Sybil-proofness, we also consider cost
minimization for the requester and agents' collusions.
