6 ( ∑ i = 1 k α i λ i 3 ) − ( 3 ( 2 ( ∑ i = 1 k α i λ i 2 ) − ( ∑ i = 1 k α i λ i ) 2 ) + ( ∑ i = 1 k α i λ i ) 2 ) ( ∑ i = 1 k α i λ i ) ( 2 ( ∑ i = 1 k α i λ i 2 ) − ( ∑ i = 1 k α i λ i ) 2 ) 3 2