In two publications ( 1923 and 1925 ) Stepanov gave necessary and sufficient conditions for a function of two variables, defined on a set S of measure greater than zero, to have a total differential almost everywhere on S . He also worked on dynamical systems ( extending the work of George Birkhoff ), the qualitative theory of ordinary differential equations, and almost periodic functions ( extending the work of Harald Bohr ).
22.
In 1985 two six-pulse valve groups of thyristors were added to increase the voltage to 500 kV per pole for a total differential voltage of 1, 000 kV . In 1989 the station was extended on the field east of the highway ( called Sylmar East Converter Station ), where two new 1, 100 ampere, 500 kV 12-pulse thyristor converters were added in parallel with the two existing converters, giving a total transmission power of 3, 100 megawatts ( 3, 100 Amperes at + /-500 kV
