To counter malicious peers in P2P systems, PeerTrust-like schemes take advantage of similarity between peers to compute service provider's trust value. To derive the similarity, there currently exist some approaches documented which however mostly are interested in algorithm or process enhancement, the analytic study in theory for these PeerTrust-like mechanisms might be missed. This paper focuses on the basic problems in PeerTrust-like trust schemes and makes the following distinctive contributions. It formalizes the generic framework of trust mechanisms in a probabilistic manner; gives the mathematical description for PeerTrust-like trust mechanisms; attempts to figure out some unclear questions: based on PeerTrust-like schemes, how to compute similarity via Euclidean distance; should all feedback be counted in; how to compare PeerTrust-like mechanisms with other distance/similarity computations.