EN
Consider the sequence $(Cₙ)_{n≥1}$ of positive numbers defined by C₁ = 1 and $C_{n+1} = 1 + Cₙ²/4$, n = 1,2,.... Let M be a real-valued martingale and let S(M) denote its square function. We establish the bound
𝔼 |Mₙ|≤ Cₙ𝔼 Sₙ(M), n=1,2,...,
and show that for each n, the constant Cₙ is the best possible.