Poly-instanton axion inflation
  We investigate the axion inflation model derived by poly-instanton effects in
type II superstring theories. Poly-instanton effects are instanton effects
corrected by another instanton and it can generate the modulus-axion potential
with the double exponential function. Although the axion has a period of small
value, this potential can have a flat region because its derivatives are
exponentially suppressed by non-perturbative effects. From the view point of
the cosmic inflation, such potential is interesting. In this paper, we
numerically study the possibilities for realizing the cosmic inflation. We also
study their spectral index and other cosmological observables, numerically.
