Markus Jakobsson e Ari Juels, Proofs of Work and Bread Pudding Protocols, in Communications and Multimedia Security, Kluwer Academic Publishers, 1999, pp. 258-272. This paper formalizes the idea of a proof of work (POW) and introduces "the dependent idea of a bread pudding protocol", a "re-usable proof of work" (RPOW) system.