√ (c2 - v2)

c

ζ = a --------------- z .

√ (c2 - v2)

If for x′, we substitute its value x - tv, we obtain

v.c

τ = φ (v). β (t - ------- ,

c2

ξ = φ (v). β (x - vt) ,

η = φ (v) y