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In this paper, we propose a timestamp series approach to defend against Sybil attack in a vehicular ad hoc network (VANET) based on roadside unit support. The proposed approach targets the initial deployment stage of VANET when basic roadside unit (RSU) support infrastructure is available and a small fraction of vehicles have network communication capability. Unlike previously proposed schemes that require a dedicated vehicular public key infrastructure to certify individual vehicles, in our approach RSUs are the only components issuing the certificates. Due to the differences of moving dynamics among vehicles, it is rare to have two vehicles passing by multiple RSUs at exactly the same time. By exploiting this spatial and temporal correlation between vehicles and RSUs, two messages will be treated as Sybil attack issued by one vehicle if they have the similar timestamp series issued by RSUs. The timestamp series approach needs neither vehicular-based public-key infrastructure nor Internet accessible RSUs, which makes it an economical solution suitable for the initial stage of VANET.