Code to calculate the Inverse Error function using a power series approximation. Based on information at http://en.wikipedia.org/wiki/Error_function#Inverse_functions. Implemented as a method in the Math module, but it could easily be run independently.

InverseError.rb

module Math
  def self.invErf(x)
             c = []
	     c[0] = 1.0
	     c[1] = 1.0
	     result = 0.0
	     (0..100).each do |k|
	     	# Calculate C sub k
		if k > 1 then 
		   c[k] = 0.0
		   (0..(k-1)).each do |m|
			term = (c[m]*c[k-1.0-m])/((m+1.0)*(2.0*m+1.0))
			c[k] += term
		    end
		end
		result += (c[k]/(2.0*k+1))*((Math.sqrt(Math::PI)/2)*x)**(2.0*k+1)
	     end
	     return result
	 end 
end