Fun with Fixed Points

Today we’re going to have fun with fixed points. A fixed point of a function f is the value x such that f(x) = x. Not everyone function has a fixed point. For example, f(x) = x + 1 has no fixed point. But many functions do. Cosine is one such function. Today we’re going to write code that finds the fixed point of the cosine function.

The procedure is basically this:

Pick a number a. I’ll start with 1.0.
Check whether a is about the same as cos(a). If it is, return it.
Set a := cos(a) and repeat.

Here’s the procedure in Ruby:

	TOLERANCE = 0.00000000000000000001

	def tolerance_equals?(a, b)
	((a – b) * (a – b)) < TOLERANCE
	end

	def recursive_cosine_fixedpoint(a)
	return a if tolerance_equals?(a, Math.cos(a))
	recursive_cosine_fixedpoint(Math.cos(a))
	end

	puts "The fixed point of cosine is #{recursive_cosine_fixedpoint(1)}"
	# The fixed point of cosine is 0.7390851331706995

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ruby-cosine-fixedpoint.rb

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And because we’re feeling whimsical, we’re going to do the same thing in Python:

	import math

	tolerance = 0.00000000000000000001

	def tolerance_equals(a, b):
	return ((a – b) * (a – b)) < tolerance

	def recursive_cosine_fixedpoint(a):
	if tolerance_equals(a, math.cos(a)):
	return a
	return recursive_cosine_fixedpoint(math.cos(a))


	print("The fixed point of cosine is {}".format(recursive_cosine_fixedpoint(1)))
	# The fixed point of cosine is 0.7390851331706995

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python-cosine-fixedpoint.py

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And Rust:

	const TOLERANCE: f64 = 0.00000000000000000001;

	fn tolerance_equals(a: f64, b: f64) -> bool {
	((a – b) * (a – b)) < TOLERANCE
	}

	fn recursive_cosine_fixedpoint(a: f64) -> f64 {
	if tolerance_equals(a, a.cos()) {
	a
	} else {
	recursive_cosine_fixedpoint(a.cos())
	}
	}

	fn main() {
	println!("The fixed point of cosine is {}", recursive_cosine_fixedpoint(1f64));
	}
	// The fixed point of cosine is 0.7390851331706995