x=A\b triggers a bug in MATLAB R2009a and earlier
Name TS
Group MathWorks
Matrix ID 2256
Num Rows 2,142
Num Cols 2,142
Nonzeros 45,262
Pattern Entries 45,262
Kind Counter Example Problem
Symmetric No
Date 2009
Author P. Quillen
Editor T. Davis
Structural Rank 2,140
Structural Rank Full false
Num Dmperm Blocks 92
Strongly Connect Components 183
Num Explicit Zeros 0
Pattern Symmetry 95.3%
Numeric Symmetry 4.1%
Cholesky Candidate no
Positive Definite no
Type real
SVD Statistics
Matrix Norm 4.434200e+02
Minimum Singular Value 1.961143e-15
Condition Number 2.261028e+17
Rank 2,140
sprank(A)-rank(A) 0
Null Space Dimension 2
Full Numerical Rank? no
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This is a structurally singular matrix that triggers a bug in the   
2-by-2 pivoting strategy in UMFPACK versions 4.1 through 5.3.       
The bug thus exists in MATLAB R2006b through R2009a.  It was        
fixed in UMFPACK 5.4 and MATLAB R2009b.  Contributed by Pat Quillen,
The MathWorks, Inc.  The sprank(A) and rank(A) are 2140.