Matrices contributed by The Mathworks, Inc.
Some matrices come from other sources.

pivtol: A small matrix from the spline toolbox that illustrates the dramatic
    growth factor that can occur with a pivot tolerance of 0.1 (in
    UMFPACK v4.0).  Needs a tolerance of 0.26, for off-diagonal pivoting.

Pd: An unsymmetric matrix (x=P\d) with many sparse right-hand-sides.  The Pd.A
    matrix is 8081-by-8081.  Pd.b and Pd_rhs.A are the same 8081-by-12406 matrix
    (the right-hand-sides).  It is highly reducible to block triangular form,
    with 7877 diagonal blocks, the largest of which is 30-by-30.  Only 9 of
    the blocks are bigger than 1-by-1.  This is a good candidate for a solver
    based on dmperm.

Harvard500: Web connectivity matrix from Cleve Moler
    (Numerical Computing with MATLAB, 2004).  A(i,j) is nonzero if there
    is a link from page j to page i.  The URL of page j is in Problem.url{j}
    in the MATLAB form of this problem.  For more details, see Cleve Moler's
    book at http://www.mathworks.com/moler .  The matrix is 500-by-500, with
    a structural rank of 233 and numerical rank of 170.

tomography:  An earth science problem from David Yang.  Contributed by
    Bobby Cheng.  Much like a medical tomography problem.  "UMFPACK 4.0
    is consistently slower than MATLAB v5."  In MATLAB 7.1,
    [L,U,P] = lu (A) takes 0.07 seconds (using GPLU), but UMFPACK v4.3 with
    [L,U,P,Q] = lu (A) takes 0.11 seconds.

Kuu, Muu:  symmetric positive definite matrices which caused problems with
    UMFPACK v4.0 (in MATLAB 6.5).  chol, or UMFPACK v4.3 or later, work fine.