Small-scale structure and 21cm fluctuations by primordial black holes
  We discuss early structure formation of small scales sourced by primordial
black holes (PBHs) which constitute a small part of present cold dark matter
component. We calculate the mass function and power spectrum of haloes
originated from the Poisson fluctuations of PBH number and show that the number
of small haloes is significantly modified in the presence of PBHs even if their
fraction accounts for only $10^{-4}$--$10^{-3}$ of total dark matter abundance.
We then compute the subsequent 21cm signature from those haloes. We find that
PBHs can provide major contributions at high redshifts within the detectability
of future experiments such as Square Kilometer Array, and provide a forecast
constraint on the PBH fraction.
