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Using sage to check if a vector is in a matrix's null space

asked 2013-11-26 13:04:34 +0200

ensaba gravatar image

I want to check if a given vector is inside a matrix's null space.

Here is what I have done:

#Define a matrix

#Free variables to be used in the given vector
var('s t')

#The null space of the matrix A


#Create a vector

#If ans is in the null space of A, then A*ans is the 0 vector
print A*ans

#If A*ans is the 0 vector, then the following should be true too
print ans in ns

I had expected the last print statement to be true since A*ans results in a zero vector, hence ans is in the null space of A.

Why isn't this the case?

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answered 2013-11-26 13:17:44 +0200

updated 2013-11-26 13:18:45 +0200

Your vector ans is not in A.kernel() because of how A.kernel() is defined. From its documentation:

Returns the left kernel of this matrix, as a vector space or free
module. This is the set of vectors "x" such that "x*self = 0".

Note: For the right kernel, use "right_kernel()".  The method
"kernel()" is exactly equal to "left_kernel()".

So since ans * A is not zero, ans is not in A.kernel().

sage: ans in A.kernel()
sage: ans in A.right_kernel()

A.right_kernel() is what you want.

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answered 2013-11-26 13:14:35 +0200

tmonteil gravatar image

Actually, the .kernel() method corresponds to the left kernel. What you are looking for is the right kernel:

sage: ns = A.right_kernel()
sage: ans in ns
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Asked: 2013-11-26 13:04:34 +0200

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Last updated: Nov 26 '13