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# umf_lufact

lu factorisation of a sparse matrix

### Calling Sequence

LU_ptr = umf_lufact(A)

### Arguments

- A
a sparse, real or complex, square or rectangular, matrix

- LU_ptr
a pointer to umf lu factors (L,U,p,q,R)

### Description

This function computes a LU factorisation of the sparse matrix A () and return at the scilab level, a pointer (LU_ptr) to an handle of the LU factors (L,U,p,q,R) (the memory used for them is "outside" scilab stack).

This function must be used in place of umfpack if you have multiple linear systems with the same matrix to solve when the rhs are not known at the same time (for instance A x1 = b1 and A x2 = b2 but b2 depends on x1, etc...).

When such a
factorisation have been computed, a linear system must be solved
with umf_lusolve (in general x = umf_lusolve(LU_ptr, b) but others
options are possible, see umf_lusolve.
**To free the memory used by the LU factors, use umf_ludel(LU_ptr) (umf_ludel);**
to retrieve the LU factors at the scilab level (for example to display their sparse
patterns), use umf_luget; to get some information (number of
non zeros in L and U), use umf_luinfo.
To compute an approximation of the condition number use condestsp

### Examples

// this is the small linear test system from UMFPACK // whom solution must be [1;2;3;4;5] A = sparse( [ 2 3 0 0 0; 3 0 4 0 6; 0 -1 -3 2 0; 0 0 1 0 0; 0 4 2 0 1] ); b = [8 ; 45; -3; 3; 19]; Lup = umf_lufact(A); x = umf_lusolve(Lup,b) // solve now A'x=b x = umf_lusolve(Lup,b,"A''x=b") norm(A'*x - b) // do not forget to clear memory with umf_ludel(Lup) // a real (but small) example // first load a sparse matrix [A] = ReadHBSparse(SCI+"/modules/umfpack/examples/arc130.rua"); // compute the factorisation Lup = umf_lufact(A); b = rand(size(A,1),1); // a random rhs // use umf_lusolve for solving Ax=b x = umf_lusolve(Lup,b); norm(A*x - b) // now the same thing with iterative refiment x = umf_lusolve(Lup,b,"Ax=b",A); norm(A*x - b) // solve now the system A'x=b x = umf_lusolve(Lup,b,"A''x=b"); // without refinement norm(A'*x - b) x = umf_lusolve(Lup,b,"A''x=b",A); // with refinement norm(A'*x - b) // do not forget to clear memory umf_ludel(Lup)

### See Also

- umfpack — solve sparse linear system
- umf_luget — retrieve lu factors at the scilab level
- umf_lusolve — solve a linear sparse system given the LU factors
- umf_ludel — utility function used with umf_lufact
- umf_luinfo — get information on LU factors
- condestsp — estimate the condition number of a sparse matrix

### Authors

- umfpack by Timothy A. Davis (see umf_license)
- scilab interface by Bruno Pincon with contributions from Antonio Frasson

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