ω₄ = i/√(1 - u²).
(40) where ω₁² + ω₂² + ω₃² + ω₄² = -1 and -iω₄ > 0.
By F and f shall be understood the space time vectors of the second kind M - iE, m - ie.
In Φ = ωF, we have a space time vector of the first kind with components
Φ₁ = ω₂F₁₂ + ω₃F₁₃ + ω₄F₁₄
Φ₂ = ω₁F₂₁ + ω₃F₂₃ + ω₄F₂₄
Φ₃ = ω₁F₃₁ + ω₂F₃₂ + ω₄F₃₄
Φ₄ = ω₁F₄₁ + ω₂F₄₂ + ω₃F₄₃
The first three quantities (φ₁, φ₂, φ₃) are the components of the space-vector (E + [u, M])/√(1 - u²),
and further (φ₄ = i[u E]/√(1 - u²).