

   TThhee PPooiissssoonn DDiissttrriibbuuttiioonn

        dpois(x, lambda)
        ppois(q, lambda)
        qpois(p, lambda)
        rpois(n, lambda)

   AArrgguummeennttss::

          x: vector of (positive) quantiles.

          p: vector of probabilities.

          n: number of random values to return.

     lambda: vector of positive means.

   DDeessccrriippttiioonn::

        These functions provide information about the Poisson
        distribution with parameter `lambda'.  `dpois' gives
        the density, `ppois' gives the distribution function
        `qpois' gives the quantile function and `rpois' gener-
        ates random deviates.

        The Poisson distribution has density

                    p(x) = lambda^x exp(-lambda)/x!

        for x = 0, 1, 2, ....

   EExxaammpplleess::

        -log(dpois(0:7, lambda=1) * gamma(1+ 0:7))
        Ni <- rpois(50, lam= 4); table(factor(Ni, 0:max(Ni)))

