1 files changed, 200 insertions, 0 deletions
diff --git a/libm/double/chdtr.c b/libm/double/chdtr.c
new file mode 100644
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+/*							chdtr.c
+ *
+ *	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.
+ *
+ *
+ *                                  inf.
+ *                                    -
+ *                        1          | |  v/2-1  -t/2
+ *  P( x | v )   =   -----------     |   t      e     dt
+ *                    v/2  -       | |
+ *                   2    | (v/2)   -
+ *                                   x
+ *
+ * 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 igam().
+ *
+ * ERROR MESSAGES:
+ *
+ *   message         condition      value returned
+ * chdtr domain   x < 0 or v < 1        0.0
+ */
+/*							chdtrc()
+ *
+ *	Complemented Chi-square distribution
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * double v, x, y, chdtrc();
+ *
+ * y = chdtrc( v, x );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Returns the area under the right hand tail (from x to
+ * infinity) of the Chi square probability density function
+ * with v degrees of freedom:
+ *
+ *
+ *                                  inf.
+ *                                    -
+ *                        1          | |  v/2-1  -t/2
+ *  P( x | v )   =   -----------     |   t      e     dt
+ *                    v/2  -       | |
+ *                   2    | (v/2)   -
+ *                                   x
+ *
+ * where x is the Chi-square variable.
+ *
+ * The incomplete gamma integral is used, according to the
+ * formula
+ *
+ *	y = chdtr( v, x ) = igamc( v/2.0, x/2.0 ).
+ *
+ *
+ * The arguments must both be positive.
+ *
+ *
+ *
+ * ACCURACY:
+ *
+ * See igamc().
+ *
+ * ERROR MESSAGES:
+ *
+ *   message         condition      value returned
+ * chdtrc domain  x < 0 or v < 1        0.0
+ */
+/*							chdtri()
+ *
+ *	Inverse of complemented Chi-square distribution
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * double df, x, y, chdtri();
+ *
+ * x = chdtri( df, y );
+ *
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Finds the Chi-square argument x such that the integral
+ * from x to infinity of the Chi-square density is equal
+ * to the given cumulative probability y.
+ *
+ * This is accomplished using the inverse gamma integral
+ * function and the relation
+ *
+ *    x/2 = igami( df/2, y );
+ *
+ *
+ *
+ *
+ * ACCURACY:
+ *
+ * See igami.c.
+ *
+ * ERROR MESSAGES:
+ *
+ *   message         condition      value returned
+ * chdtri domain   y < 0 or y > 1        0.0
+ *                     v < 1
+ *
+ */
+
+/*								chdtr() */
+
+
+/*
+Cephes Math Library Release 2.8:  June, 2000
+Copyright 1984, 1987, 2000 by Stephen L. Moshier
+*/
+
+#include <math.h>
+#ifdef ANSIPROT
+extern double igamc ( double, double );
+extern double igam ( double, double );
+extern double igami ( double, double );
+#else
+double igamc(), igam(), igami();
+#endif
+
+double chdtrc(df,x)
+double df, x;
+{
+
+if( (x < 0.0) || (df < 1.0) )
+	{
+	mtherr( "chdtrc", DOMAIN );
+	return(0.0);
+	}
+return( igamc( df/2.0, x/2.0 ) );
+}
+
+
+
+double chdtr(df,x)
+double df, x;
+{
+
+if( (x < 0.0) || (df < 1.0) )
+	{
+	mtherr( "chdtr", DOMAIN );
+	return(0.0);
+	}
+return( igam( df/2.0, x/2.0 ) );
+}
+
+
+
+double chdtri( df, y )
+double df, y;
+{
+double x;
+
+if( (y < 0.0) || (y > 1.0) || (df < 1.0) )
+	{
+	mtherr( "chdtri", DOMAIN );
+	return(0.0);
+	}
+
+x = igami( 0.5 * df, y );
+return( 2.0 * x );
+}