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diff --git a/libgo/go/math/atan.go b/libgo/go/math/atan.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
+
+/*
+	Floating-point arctangent.
+
+	Atan returns the value of the arctangent of its
+	argument in the range [-pi/2,pi/2].
+	There are no error returns.
+	Coefficients are #5077 from Hart & Cheney. (19.56D)
+*/
+
+// xatan evaluates a series valid in the
+// range [-0.414...,+0.414...]. (tan(pi/8))
+func xatan(arg float64) float64 {
+	const (
+		P4 = .161536412982230228262e2
+		P3 = .26842548195503973794141e3
+		P2 = .11530293515404850115428136e4
+		P1 = .178040631643319697105464587e4
+		P0 = .89678597403663861959987488e3
+		Q4 = .5895697050844462222791e2
+		Q3 = .536265374031215315104235e3
+		Q2 = .16667838148816337184521798e4
+		Q1 = .207933497444540981287275926e4
+		Q0 = .89678597403663861962481162e3
+	)
+	sq := arg * arg
+	value := ((((P4*sq+P3)*sq+P2)*sq+P1)*sq + P0)
+	value = value / (((((sq+Q4)*sq+Q3)*sq+Q2)*sq+Q1)*sq + Q0)
+	return value * arg
+}
+
+// satan reduces its argument (known to be positive)
+// to the range [0,0.414...] and calls xatan.
+func satan(arg float64) float64 {
+	if arg < Sqrt2-1 {
+		return xatan(arg)
+	}
+	if arg > Sqrt2+1 {
+		return Pi/2 - xatan(1/arg)
+	}
+	return Pi/4 + xatan((arg-1)/(arg+1))
+}
+
+// Atan returns the arctangent of x.
+//
+// Special cases are:
+//	Atan(±0) = ±0
+//	Atan(±Inf) = ±Pi/2
+func Atan(x float64) float64 {
+	if x == 0 {
+		return x
+	}
+	if x > 0 {
+		return satan(x)
+	}
+	return -satan(-x)
+}