quantiles of the Chi-Square function at gamma with n degrees of
freedom.
Approximation according to Wilson and Hilferty.
Note: not reliable for small n (probably because it depends on the
inaccurate gauss_quant_std function ...)
returns the Gaussian response at x.
opt is the optimum response (corresponds to mu)
tol is the tolearance of the response (corresponds to sigma)
ampl is the maximum response
computes the density of the bivariate normal distribution at point
(x,`y`). x and y are presumed to be independent (i.e., their
correlation is 0).
See Hartung J(1986): Statistik. Oldenbourg, Muenchen. p. 120.
returns the value of a bivariate Gaussian response curve at point
(x,`y`). x and y are presumed to be independent.
opt_* is the x-value of the optimum response (corresponds to mu),
tol_* is the tolerance of the response (corresponds to sigma),
ampl is the maximum reponse.
See Jongman et al.(1987): Data analysis in community and landscape ecology.
Pudoc, Wageningen. pp. 40ff.