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I am looking for a function that could replace matrices composed of scalar matrices subparts with matrices where those subparts are replaced with numbers.

That is,

$$\left( \begin{array}{cccc} 1 & 0 & 2 & 0 \\ 0 & 1 & 0 & 2 \\ 3 & 0 & 4 & 0 \\ 0 & 3 & 0 & 4 \\ \end{array} \right)\to \left( \begin{array}{cc} 1 & 2 \\ 3 & 4 \\ \end{array} \right)$$

That is, an operation inverse to KroneckerProduct[A, IdentityMatrix[n]] (which replaces numbers with scalar matrices of order n).

The function should look for the best replacement possible, that is the result should be a matrix of minimal possible order.

How to realise this?

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  • $\begingroup$ I would look at the paper Van Loan, Charles F. "The ubiquitous Kronecker product." (core.ac.uk/download/pdf/82128039.pdf). I think it discusses the solution of such of equations. $\endgroup$
    – mikado
    Dec 11, 2022 at 14:38

1 Answer 1

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BlockMap and Tr.

n=2;
A = {{1, 2}, {3, 4}};
B = KroneckerProduct[A, IdentityMatrix[n]];
BlockMap[Tr, B, {n, n}]/n
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  • $\begingroup$ And how the code would determine the suitable n? $\endgroup$
    – Anixx
    Dec 11, 2022 at 8:00
  • $\begingroup$ The matrices that do not allow such simplification should not be affected, and those which allow several ways should choose the best simplification. $\endgroup$
    – Anixx
    Dec 11, 2022 at 8:02
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    $\begingroup$ @Anixx We needs an example to illustrate such complex cases. $\endgroup$
    – cvgmt
    Dec 11, 2022 at 8:15
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    $\begingroup$ @Anixx Edit the original question and add such examples can help other people to read the comple question. $\endgroup$
    – cvgmt
    Dec 11, 2022 at 9:27
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    $\begingroup$ squeeze[x_] := SelectFirst[BlockMap[Tr, x, {#, #}]/# & /@ Divisors[Length[x]] // Reverse, KroneckerProduct[#, IdentityMatrix[Length[x]/Length[#]]] == x &] $\endgroup$
    – bartyslartfast
    Dec 12, 2022 at 17:01

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