On the formation of spiral arms in dwarf galaxies
  Spiral structure (both flocculent and Grand Design types) is very rarely
observed in dwarf galaxies because the formation of spiral arms requires
special conditions. In this work we analyze the sample of about 40 dS-galaxies
found by scanning by eye the images of late-type galaxies with $m_B<15^m$ and
$M_B>-18^m$ and photometric diameter $D_{25}<12$~kpc. We found that apart from
the lower average gas (HI) fraction the other properties of dS-galaxies
including the presence of a bar and the isolation index do not differ much from
those for dwarf Irr or Sm-types of similar luminosity and rotation velocity (or
specific angular momentum).There are practically no dS-galaxies with rotation
velocity below 50\,--\,60~km\,sec$^{-1}$.
  To check the conditions of formation of spiral structure in dwarf galaxies we
carried out a series of N-body/hydrodynamic simulations of low-mass
stellar-gaseous discy galaxies by varying the model kinematic parameters of
discs, their initial thickness, relative masses and scale lengths of stellar
and gaseous disc components, and stellar-to-dark halo masses. We came to
conclusion that the gravitational mechanism of spiral structure formation is
effective only for thin stellar discs, which are non-typical for dwarf
galaxies, and for not too slowly rotating galaxies. Therefore, only a small
fraction of dwarf galaxies with stellar/gaseous discs have spiral or ring
structures. The thicker stellar disc, the more gas is required for the spiral
structure to form. The reduced gas content in many dS-galaxies compared to
non-spiral ones may be a result of more efficient star formation due to a
higher volume gas density thank to the thinner stellar/gaseous discs.
