Actually, the paper is using a very well known, studied, and used definition of efficiency - weak form efficiency. This is what current economists use when the

Actually, the paper is using a very well known, studied, and used definition of efficiency - weak form efficiency. This is what current economists use when they talk about markets being efficient, so it makes sense for the paper to use it as well. There are alternative definitions (strong, semi-strong), both of which have been shown to be mathematically false already (long before economists stopped believing them, sadly)