The famous mass formula was already discussed by Pointcarre and some German physicist. You might say that it was in the air at the time.

JES

Samuel Reich Following on from the work of Maxwell there was a proposal for length contraction proposed by Fitzgerald to explain the Michelson Morley experiment.

This used the factor (1 - (v/c)^2)^0.5

This predated Einstein by some years to the late 1800s.

Richard

Richard Lewis

Length contraction is not mass growth. It was Einstein presented a mathematical proof of m=mo 1/[(1-(v/c)^2]^0.5 and expanded the Lorentz transformation to change observed time. But to formula the observed time and space change Lorentz and Einstein had get have the Axial Doppler shift disappear. I do not read German so I am not sure what erroneous assumption they used to get the disappearance. Two American on relativity got a sign wrong in velocity in the Axial Doppler shift. That velocity is the relative velocity of source to the observer the sign only changes if they go from moving toward each other to moving away from each other. If the light bounces off one the sign of the relative velocity of source to the observed does not change.

In any case the attached files show what happens when the Axial shift is not deleted and therefore changes observed time just as the transverse shift does. It gives proofs for that independent MM experiment. One is based on the properties of the Fourier time series (not transform)/