Compute the distribution function of Inverse gamma distribution
pinvGamma(q, a, b, lower.tail = TRUE, log.p = FALSE)
vector of quantiles
the shape parameter of the inverse gamma distribution
the scale parameter of the inverse gamm distribution
logical; if TRUE (default), probabilities are P[X > x], otherwise, P[X <= x].
logical; if FALSE (default) return the probability, otherwise return the natural logarithm of the probability