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2015-03-30 10:39:01 +0100 asked a question How to use DiscreteRandomVariable covariance method?

I'd like to take advantage of covariance() method of DiscreteRandomVariable class, yet, I can't find any example on the internet how to properly use it.

My understanding is that this should work:

text1 = AlphabeticStrings().encoding("AB")
text2 = AlphabeticStrings().encoding("BA")
text1freq = frequency_distribution(text1)
text2freq = frequency_distribution(text2)
text1freq.covariance(text2freq)

However, I receive unexpected ValueError:

/usr/lib/python2.7/site-packages/sage/probability/random_variable.pyc in covariance(self, other)
187         Omega = self.probability_space()
188         if Omega != other.probability_space():
--> 189             raise ValueError("Argument other (= %s) must be defined on the same probability space." % other)
190         muX = self.expectation()
191         muY = other.expectation()

ValueError: Argument other (= Discrete probability space defined by {A: 0.500000000000000, B: 0.500000000000000}) must be defined on the same probability space.

But... Logically (not technically) both text1freq and text2freq are the same probability space:

sage: text1freq
Discrete probability space defined by {A: 0.500000000000000, B: 0.500000000000000}
sage: text2freq
Discrete probability space defined by {A: 0.500000000000000, B: 0.500000000000000}