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If I type into Mathematica

TensorProduct[IdentityMatrix[2],IdentityMatrix[2]]

It gives me a result that has nested matrices. How do I turn that into a normal matrix without any nesting? Thanks

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    $\begingroup$ Look at Flatten, Join, et al. ... $\endgroup$ – ciao Jul 2 '14 at 2:32
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Depends on what dimension your final matrix is supposed to have. When I should make a guess, I would say you want this

TensorProduct[IdentityMatrix[2], IdentityMatrix[2]] // ArrayFlatten

Mathematica graphics

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An alternative approach is to use KroneckerProduct which does not require flattening.

From the docs on TensorProduct:

The KroneckerProduct of matrices is equivalent to the flattening of their TensorProduct to another matrix.

That is, for any two matrices m1 and m2

KroneckerProduct[m1, m2] == ArrayFlatten[TensorProduct[m1, m2]]
(* True *)

So,

m1 = IdentityMatrix[2];
KroneckerProduct[m1, m1] == ArrayFlatten[TensorProduct[m1, m1]]
(* True *)
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  • $\begingroup$ Excellent. I've been looking to define such a function. Is there a convenient symbol for it like with the Tensor Product: esc t* esc ? $\endgroup$ – Travis Bemrose Jul 12 '16 at 20:10
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    $\begingroup$ @Travis, maybe you can define an input alias such as CurrentValue[$FrontEndSession, {InputAliases, "KrP"}] = RowBox[{"KroneckerProduct[", "\[SelectionPlaceholder]", ",", "\[Placeholder]", "]"}] and use it as Esc KrP Esc to get a template with placeholders. Or, define CirclePlus = KroneckerProduct and us it as matrix1 Esc c+ Esc matrix2. $\endgroup$ – kglr Jul 12 '16 at 20:33

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