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+/* iv.c
+ *
+ * Modified Bessel function of noninteger order
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * double v, x, y, iv();
+ *
+ * y = iv( v, x );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Returns modified Bessel function of order v of the
+ * argument. If x is negative, v must be integer valued.
+ *
+ * The function is defined as Iv(x) = Jv( ix ). It is
+ * here computed in terms of the confluent hypergeometric
+ * function, according to the formula
+ *
+ * v -x
+ * Iv(x) = (x/2) e hyperg( v+0.5, 2v+1, 2x ) / gamma(v+1)
+ *
+ * If v is a negative integer, then v is replaced by -v.
+ *
+ *
+ * ACCURACY:
+ *
+ * Tested at random points (v, x), with v between 0 and
+ * 30, x between 0 and 28.
+ * Relative error:
+ * arithmetic domain # trials peak rms
+ * DEC 0,30 2000 3.1e-15 5.4e-16
+ * IEEE 0,30 10000 1.7e-14 2.7e-15
+ *
+ * Accuracy is diminished if v is near a negative integer.
+ *
+ * See also hyperg.c.
+ *
+ */
+ /* iv.c */
+/* Modified Bessel function of noninteger order */
+/* If x < 0, then v must be an integer. */
+
+
+/*
+Cephes Math Library Release 2.8: June, 2000
+Copyright 1984, 1987, 1988, 2000 by Stephen L. Moshier
+*/
+
+
+#include <math.h>
+#ifdef ANSIPROT
+extern double hyperg ( double, double, double );
+extern double exp ( double );
+extern double gamma ( double );
+extern double log ( double );
+extern double fabs ( double );
+extern double floor ( double );
+#else
+double hyperg(), exp(), gamma(), log(), fabs(), floor();
+#endif
+extern double MACHEP, MAXNUM;
+
+double iv( v, x )
+double v, x;
+{
+int sign;
+double t, ax;
+
+/* If v is a negative integer, invoke symmetry */
+t = floor(v);
+if( v < 0.0 )
+ {
+ if( t == v )
+ {
+ v = -v; /* symmetry */
+ t = -t;
+ }
+ }
+/* If x is negative, require v to be an integer */
+sign = 1;
+if( x < 0.0 )
+ {
+ if( t != v )
+ {
+ mtherr( "iv", DOMAIN );
+ return( 0.0 );
+ }
+ if( v != 2.0 * floor(v/2.0) )
+ sign = -1;
+ }
+
+/* Avoid logarithm singularity */
+if( x == 0.0 )
+ {
+ if( v == 0.0 )
+ return( 1.0 );
+ if( v < 0.0 )
+ {
+ mtherr( "iv", OVERFLOW );
+ return( MAXNUM );
+ }
+ else
+ return( 0.0 );
+ }
+
+ax = fabs(x);
+t = v * log( 0.5 * ax ) - x;
+t = sign * exp(t) / gamma( v + 1.0 );
+ax = v + 0.5;
+return( t * hyperg( ax, 2.0 * ax, 2.0 * x ) );
+}