1. Let {X 0 , X 1 , ...} be the discrete Black-Scholes model defined by Where Y 0 is an arbitrary positive random variable with finite mean, and Y i  = e Z i with independent Z i = N(μ,σ 2 ); i = 1,...

1. Let {X₀, X₁, ...} be the discrete Black-Scholes model defined by

Where Y₀
is an arbitrary positive random variable with finite mean, and Y_i
 = e^Zi with independent Z_i
= N(μ,σ²); i = 1, 2, ... Under which condition is {X₀, X₁, ...} a martingale?