chdtr

Chi-square distribution.

SYNOPSIS:

double df, x, y, chdtr();
y = chdtr(df, x);

DESCRIPTION:

Returns the area under the left hand tail (from 0 to x) of the Chi square probability density function, with v degrees of freedom.

                                   x
                                  -
                      1          | | v/2-1 -t/2
P(x | v)   =   -----------     |   t      e     dt
                  v/2 -       | |
                 2    | (v/2)   -
                                 0

where x is the Chi-square variable.

The incomplete gamma integral is used, according to the formula:

y = chdtr(v, x) = igam(v/2.0, x/2.0).

The arguments must both be positive.

ACCURACY:

See the igam() Help topic.

ERROR MESSAGES:

message       condition       value returned
domain       x < 0 or v < 1        0.0