The variance of returns on a portfolio with proportions x i {\displaystyle x_{i}} invested in stock i {\displaystyle i} is:
∑ i = 1 N ∑ j = 1 N x i x j σ i j {\displaystyle \sum _{i=1}^{N}\sum _{j=1}^{N}{x_{i}}{x_{j}}\sigma _{ij}}
