Which of the following two formulas is the correct wavelength expression of a relativistic electron? $$
\lambda=\dfrac{h}{m\gamma v }
\tag{01}\label{01}
$$
or
$$
\lambda=\dfrac{hv}{m\gamma c^2 }
\tag{02}\label{02}
$$
Edit:
The energy of an electron is, by special relativity
$$
E=\gamma mc^2
\tag{03}\label{03}
$$
where
$$
\gamma = \left(1-\frac{v^2}{c^2}\right)^{-1/2}
\tag{04}\label{04}
$$
The frequency $f$ satisfies the Planck relation:
$$
E = hf
\tag{05}\label{05}
$$
and the frequency and wavelength $\:\lambda\:$ of any wave must satisfy
$$
f\lambda  = v
\tag{06}\label{06}
$$
Put all of those together and I get
$$
\lambda = \dfrac{v}{f} = \dfrac{hv}{E} = \dfrac{hv}{\gamma mc^2}
\tag{07}\label{07}
$$
 A: Reference : My answer there About de Broglie relations, what exactly is E? Its energy of what?.
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The correct wavelength expression of a relativistic electron is (01), while (02) is wrong. This is due to the wrong equation (06). The wavelength $\:\lambda\:$ and the frequency $\:f\:$ are characteristics of the "superluminal" plane phase wave associated to the "subluminal" particle. The speed of this phase wave is $\:c^2/\upsilon$, so the correct equation (06) is
\begin{equation}
f\lambda\boldsymbol{=}\dfrac{c^2}{\upsilon}
\tag{A-01}\label{A-01}
\end{equation}
Hence the correct equation (07) is
\begin{equation}
\lambda \boldsymbol{=}\dfrac{c^2}{f\upsilon}\boldsymbol{=}\dfrac{c^2}{\left(E/h\right)\upsilon}\boldsymbol{=}\dfrac{c^2}{\left(\gamma m c^2/h\right)\upsilon}\boldsymbol{=}\dfrac{h}{\gamma m \upsilon}
\tag{A-02}\label{A-02}
\end{equation}
that is equation (01) of the question.
