  Two properties of the radiation emitted by a relativistic electron in an
external field, in the classical approximation, are presented in details: 1)
spectral sum rules and their relationship with the sum rules for oscillator
strength in atomic physics. Filtered sum rules can be used to remove infrared
or ultraviolet divergences. 2) "half-naked electron" effects like the Landau-
Pomeranchuk-Migdal and Ternovskij-Shul'ga-Fomin effects. These properties are
best understood when introducing the apparent transverse velocity.
