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Questions on the manipulation of matrices in Mathematica.

10
votes
There is an undocumented built-in solution: rules = {#, #} -> RandomReal[{}, {128, 128}] & /@ Range[48]; SparseArray`SparseBlockMatrix[rules]; // RepeatedTiming (* {0.042, Null} *)
answered Nov 16 '15 by Michael Weyrauch
2
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]; which runs rather fast. res should contain the matrix you want to produce. The various Flatten are necessary in order to bring the indices of the tensors into the appropriate order. Note that Dot … memory.) Note that I separated the sums over $i$ and $j$ which appears to be possible here. With these building blocks one can obtain the desired matrix as follows: term = (Flatten[Conjugate[bm], {{3 …
answered Mar 19 '16 by Michael Weyrauch