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Re^2: decomposing binary matrices

by hv (Parson)
 on Feb 16, 2007 at 15:16 UTC ( #600455=note: print w/replies, xml ) Need Help??

in reply to Re: decomposing binary matrices

Thanks. First, I should note that there must be at least as many values as variables, since each variable must take a distinct value within the set of possible values.

Second, variables that are part of an n-element submatrix need not have n bits set. The sparsest counterexample is:

```my @grid = (
# 0  1  2  3  4  5
[ 0, 0, 0, 1, 1, 0, ],
[ 0, 0, 0, 1, 0, 1, ],
[ 0, 0, 0, 0, 1, 1, ],
[ 1, 1, 0, 0, 0, 0, ],
[ 1, 0, 1, 0, 0, 0, ],
[ 0, 1, 1, 0, 0, 0, ],
);
.. which can decompose into two 3-element submatrices.

The least sparse version of that is:

```my @grid = (
# 0  1  2  3  4  5
[ 0, 0, 0, 1, 1, 1, ],
[ 0, 0, 0, 1, 1, 1, ],
[ 0, 0, 0, 1, 1, 0, ],
[ 1, 1, 1, 1, 1, 1, ],
[ 1, 1, 1, 1, 1, 1, ],
[ 1, 1, 0, 1, 1, 1, ],
);
.. which can decompose the same way.

Update: swapped 2 bits in the last row of the sparse matrix, so it actually represents what I'm saying

Hugo

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Re^3: decomposing binary matrices
by BrowserUk (Pope) on Feb 16, 2007 at 15:55 UTC

Hm. Sorry to have wasted your time. I'd picked up on this bit of the OP

... since A and E are restricted to only two values between them, they must consume those two values;

... and hung my hat on it, but that obviously doesn't apply in the same way to the two examples above.

Question: Would this example also decompose into the (same?) two groups as the above?

```my @grid = (
[ 0, 1, 0, 1, 0, 0, ],
[ 0, 0, 0, 1, 1, 0, ],
[ 0, 1, 0, 0, 1, 0, ],
[ 1, 0, 0, 0, 0, 1, ],
[ 1, 0, 1, 0, 0, 0, ],
[ 1, 0, 0, 0, 0, 1, ],
);

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

Yes: labelling the columns A..F and the rows 1..6, we know variables {B, D, E} must consume values {1, 2, 3} between them, and likewise {A, C, F} must consume {4, 5, 6}. In this example, however, the latter can be further decomposed: C can only be 5, so {A, F] are left with {4, 6}.

Hugo

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