We denote by [$]F(t_i, t_j)[$] the forward price for delivery day [$]t_j[$] observed on date [$]t_i[$]. $$F(t_i, t_j)=F(0, t_j)\exp\left(-\frac{1}{2}\sigma(t_i,t_j)^2t_i+\sigma(t_i,t_j)W(t_i,m)\right)$$ where - [$]m[$] is the month containing period [$]t_j[$] -[$]\sigma(t_i, t_j)[$] ...