A = {{1., 0.}, {0., 2.}};
LinearAlgebra`MatrixConditionNumber[A]
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2
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After @CarlWoll's comment:
You can search for undocumented functions by using Names
Names["*`" <> "MatrixConditionNumber" <> "*"]
(* {"LinearAlgebra`Private`MatrixConditionNumber"} *)
Now calling the correct function name
A = {{1., 0.}, {0., 2.}};
LinearAlgebra`Private`MatrixConditionNumber[A]
(* 2. *)
LinearAlgebra`Private`MatrixConditionNumber
. $\endgroup$ – Carl Woll Jul 10 '19 at 15:00