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diff --git a/src/pkg/math/exp.go b/src/pkg/math/exp.go
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+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package math
+
+import "math"
+
+// The original C code, the long comment, and the constants
+// below are from FreeBSD's /usr/src/lib/msun/src/e_exp.c
+// and came with this notice.  The go code is a simplified
+// version of the original C.
+//
+// ====================================================
+// Copyright (C) 2004 by Sun Microsystems, Inc. All rights reserved.
+//
+// Permission to use, copy, modify, and distribute this
+// software is freely granted, provided that this notice
+// is preserved.
+// ====================================================
+//
+//
+// exp(x)
+// Returns the exponential of x.
+//
+// Method
+//   1. Argument reduction:
+//      Reduce x to an r so that |r| <= 0.5*ln2 ~ 0.34658.
+//      Given x, find r and integer k such that
+//
+//               x = k*ln2 + r,  |r| <= 0.5*ln2.
+//
+//      Here r will be represented as r = hi-lo for better
+//      accuracy.
+//
+//   2. Approximation of exp(r) by a special rational function on
+//      the interval [0,0.34658]:
+//      Write
+//          R(r**2) = r*(exp(r)+1)/(exp(r)-1) = 2 + r*r/6 - r**4/360 + ...
+//      We use a special Remes algorithm on [0,0.34658] to generate
+//      a polynomial of degree 5 to approximate R. The maximum error
+//      of this polynomial approximation is bounded by 2**-59. In
+//      other words,
+//          R(z) ~ 2.0 + P1*z + P2*z**2 + P3*z**3 + P4*z**4 + P5*z**5
+//      (where z=r*r, and the values of P1 to P5 are listed below)
+//      and
+//          |                  5          |     -59
+//          | 2.0+P1*z+...+P5*z   -  R(z) | <= 2
+//          |                             |
+//      The computation of exp(r) thus becomes
+//                             2*r
+//              exp(r) = 1 + -------
+//                            R - r
+//                                 r*R1(r)
+//                     = 1 + r + ----------- (for better accuracy)
+//                                2 - R1(r)
+//      where
+//                               2       4             10
+//              R1(r) = r - (P1*r  + P2*r  + ... + P5*r   ).
+//
+//   3. Scale back to obtain exp(x):
+//      From step 1, we have
+//         exp(x) = 2^k * exp(r)
+//
+// Special cases:
+//      exp(INF) is INF, exp(NaN) is NaN;
+//      exp(-INF) is 0, and
+//      for finite argument, only exp(0)=1 is exact.
+//
+// Accuracy:
+//      according to an error analysis, the error is always less than
+//      1 ulp (unit in the last place).
+//
+// Misc. info.
+//      For IEEE double
+//          if x >  7.09782712893383973096e+02 then exp(x) overflow
+//          if x < -7.45133219101941108420e+02 then exp(x) underflow
+//
+// Constants:
+// The hexadecimal values are the intended ones for the following
+// constants. The decimal values may be used, provided that the
+// compiler will convert from decimal to binary accurately enough
+// to produce the hexadecimal values shown.
+
+// Exp returns e^x, the base-e exponential of x.
+//
+// Special cases are:
+//	Exp(+Inf) = +Inf
+//	Exp(NaN) = NaN
+// Very large values overflow to -Inf or +Inf.
+// Very small values underflow to 1.
+func Exp(x float64) float64 {
+	const (
+		Ln2Hi	= 6.93147180369123816490e-01;
+		Ln2Lo	= 1.90821492927058770002e-10;
+		Log2e	= 1.44269504088896338700e+00;
+
+		P1   =  1.66666666666666019037e-01; /* 0x3FC55555; 0x5555553E */
+		P2   = -2.77777777770155933842e-03; /* 0xBF66C16C; 0x16BEBD93 */
+		P3   =  6.61375632143793436117e-05; /* 0x3F11566A; 0xAF25DE2C */
+		P4   = -1.65339022054652515390e-06; /* 0xBEBBBD41; 0xC5D26BF1 */
+		P5   =  4.13813679705723846039e-08; /* 0x3E663769; 0x72BEA4D0 */
+
+		Overflow	= 7.09782712893383973096e+02;
+		Underflow	= -7.45133219101941108420e+02;
+		NearZero	= 1.0/(1<<28);		// 2^-28
+	)
+
+	// special cases
+	switch {
+	case IsNaN(x) || IsInf(x, 1):
+		return x;
+	case IsInf(x, -1):
+		return 0;
+	case x > Overflow:
+		return Inf(1);
+	case x < Underflow:
+		return 0;
+	case -NearZero < x && x < NearZero:
+		return 1;
+	}
+
+	// reduce; computed as r = hi - lo for extra precision.
+	var k int;
+	switch {
+	case x < 0:
+		k = int(Log2e*x - 0.5);
+	case x > 0:
+		k = int(Log2e*x + 0.5);
+	}
+	hi := x - float64(k)*Ln2Hi;
+	lo := float64(k)*Ln2Lo;
+	r := hi - lo;
+
+	// compute
+	t := r * r;
+	c := r - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
+	y := 1 - ((lo - (r*c)/(2-c)) - hi);
+	// TODO(rsc): make sure Ldexp can handle boundary k
+	return Ldexp(y, k);
+}