Now by computing the determinant 2×2: ( u − − λ λ ) ( ( u − − λ λ ) 2 − − e v v v 2 ) = 0 by defining the parameter: a ( v , s ) ≡ ≡ v e v v or equivalently in mechanical variables, as: a ( ρ ρ , p ) ≡ ≡ ∂ ∂ p ∂ ∂ ρ ρ