content_image/361418.jpg
<img src='content_image/361418.jpg'>

one expects the standard peculiarity of D ( ω ) like a finite discontinuity near the maximal frequency. For the upper band a nonstandard behavior manifests itself only at small fields h < 1 . 2, when at points like X(1,0) minima appear, while saddle points move along directions like (1,0) into the Brillouin zone. The situation here resembles that described for the FM case and we do not discuss it here. 

The scheme of Van Hove singularities for the lower branch are much less standard. First of all, for the low band at all values of field the global maximum is located at the points like X(1,0). A standard peculiarity of the density of states like a finite jump corresponds to this frequency. At h ̸ = 0 (the zero field case is special and will be considered separately) both at the edges of the Brillouin zone [points like M(1,1)] and in its center the minima are present, 

