From 1077fa4d772832f77a677ce7fb7c2d513b959e3f Mon Sep 17 00:00:00 2001
From: Eric Andersen <andersen@codepoet.org>
Date: Thu, 10 May 2001 00:40:28 +0000
Subject: uClibc now has a math library.  muahahahaha!  -Erik

---
 libm/double/jn.c | 133 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 133 insertions(+)
 create mode 100644 libm/double/jn.c

(limited to 'libm/double/jn.c')

diff --git a/libm/double/jn.c b/libm/double/jn.c
new file mode 100644
index 000000000..ee05395aa
--- /dev/null
+++ b/libm/double/jn.c
@@ -0,0 +1,133 @@
+/*							jn.c
+ *
+ *	Bessel function of integer order
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * int n;
+ * double x, y, jn();
+ *
+ * y = jn( n, x );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Returns Bessel function of order n, where n is a
+ * (possibly negative) integer.
+ *
+ * The ratio of jn(x) to j0(x) is computed by backward
+ * recurrence.  First the ratio jn/jn-1 is found by a
+ * continued fraction expansion.  Then the recurrence
+ * relating successive orders is applied until j0 or j1 is
+ * reached.
+ *
+ * If n = 0 or 1 the routine for j0 or j1 is called
+ * directly.
+ *
+ *
+ *
+ * ACCURACY:
+ *
+ *                      Absolute error:
+ * arithmetic   range      # trials      peak         rms
+ *    DEC       0, 30        5500       6.9e-17     9.3e-18
+ *    IEEE      0, 30        5000       4.4e-16     7.9e-17
+ *
+ *
+ * Not suitable for large n or x. Use jv() instead.
+ *
+ */
+
+/*							jn.c
+Cephes Math Library Release 2.8:  June, 2000
+Copyright 1984, 1987, 2000 by Stephen L. Moshier
+*/
+#include <math.h>
+#ifdef ANSIPROT
+extern double fabs ( double );
+extern double j0 ( double );
+extern double j1 ( double );
+#else
+double fabs(), j0(), j1();
+#endif
+extern double MACHEP;
+
+double jn( n, x )
+int n;
+double x;
+{
+double pkm2, pkm1, pk, xk, r, ans;
+int k, sign;
+
+if( n < 0 )
+	{
+	n = -n;
+	if( (n & 1) == 0 )	/* -1**n */
+		sign = 1;
+	else
+		sign = -1;
+	}
+else
+	sign = 1;
+
+if( x < 0.0 )
+	{
+	if( n & 1 )
+		sign = -sign;
+	x = -x;
+	}
+
+if( n == 0 )
+	return( sign * j0(x) );
+if( n == 1 )
+	return( sign * j1(x) );
+if( n == 2 )
+	return( sign * (2.0 * j1(x) / x  -  j0(x)) );
+
+if( x < MACHEP )
+	return( 0.0 );
+
+/* continued fraction */
+#ifdef DEC
+k = 56;
+#else
+k = 53;
+#endif
+
+pk = 2 * (n + k);
+ans = pk;
+xk = x * x;
+
+do
+	{
+	pk -= 2.0;
+	ans = pk - (xk/ans);
+	}
+while( --k > 0 );
+ans = x/ans;
+
+/* backward recurrence */
+
+pk = 1.0;
+pkm1 = 1.0/ans;
+k = n-1;
+r = 2 * k;
+
+do
+	{
+	pkm2 = (pkm1 * r  -  pk * x) / x;
+	pk = pkm1;
+	pkm1 = pkm2;
+	r -= 2.0;
+	}
+while( --k > 0 );
+
+if( fabs(pk) > fabs(pkm1) )
+	ans = j1(x)/pk;
+else
+	ans = j0(x)/pkm1;
+return( sign * ans );
+}
-- 
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