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I don't know if this is new to 5.1.1, and maybe I'm doing something wrong myself, but this looks like a "problem" to me. (I'll avoid the B word, for now.)

I'm attempting to retrieve the appropriate upper and lower triangle matrices from LUDecomposition and check that the multiplication actually works, so that lower.upper is a permutation of the original matrix.

The "errorSQ" function calculates data[[perm]] and lower.upper, and the difference between the two. A sum-square-difference (it should be zero) is the output. When the second argument (printFlag) is true, "errorSQ" also prints data[[perm]] and lower.upper, so that we can compare them side by side.

test[f,k,n] calls "errorSQ" for an array of n k-by-k arrays, each of them determined by f.

(There's also a check for precisely singular matrices.)

Clear[errorSQ, upperLU, lowerLU, test]
upperLU[m_?MatrixQ] := Transpose[m]Table[Boole[i <= j], {i, Length@m},
  {j, Length@m}]
lowerLU[m_?MatrixQ] := Transpose[m] - upperLU[m] + IdentityMatrix@Length@m
errorSQ[data_?MatrixQ, printFlag_:False] := Module[{m, perm, ignored,
  lower, upper, error},
     If[Det@data === 0, Infinity,
       {m, perm, ignored} = LUDecomposition@data;
       {lower, upper} = Through[{lowerLU, upperLU}@m];
       error = #.# &@Flatten[data[[perm]] - lower.upper];
       printFlag && Print /@ MatrixForm /@ {data[[perm]], lower.upper};
test[f_, k_Integer, n_Integer] := errorSQ /@ Array[Array[f, {k, k}] &, {n}]

For real-valued matrix input, I always get a difference near zero. For instance, here's the maximum squared error for 10,000 trials of 4 by 4 matrices with random real entries between -5 and 5:



But when entries are Integer (leaving out the singular cases), the error is never zero:

Min@test[Random[Integer, {-5, 5}] &, 4, 10000]


I'm generating matrices and checking results the same way in both cases (except Integer versus Real), and here's an example with printFlag=True:

errorSQ[Array[Random[Integer, {-5, 5}] &, {k, k}], True]

MatrixForm[{{-1, 5, 3, -5},
    {0, -5, -4, 2}, {1, -5, 0,
     -2}, {-5, -2, -1, 0}}]
MatrixForm[{{-1, 0, -1, 5},
    {-5, -5, -5, 152/5},
    {-3, 20, 0, -(71/15)},
    {5, -10, -16, 0}}]

data[[perm]] and lower.upper have different entries, not just the same entries permuted in some way.

So... what am I missing?


DrBob at

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