= f₃₂ f₁₄ f₁₃ f₂₄ + f₂₁ f₃₄·

4⁰. A linear transformation

xh = αh1 x₁′ + αh2 x₂′ + αh3 x₃′ + αh4 x₄′ (h = 1,2,3,

which is accomplished by the matrix

A = | α₁₁, α₁₂, α₁₃, α₁₄ |

| |

| α₂₁, α₂₂, α₂₃, α₂₄ |

| |

| α₃₁, α₃₂, α₃₃, α₃₄ |

| |