1SLASYF(1) LAPACK routine (version 3.2) SLASYF(1)
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6 SLASYF - computes a partial factorization of a real symmetric matrix A
7 using the Bunch-Kaufman diagonal pivoting method
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10 SUBROUTINE SLASYF( UPLO, N, NB, KB, A, LDA, IPIV, W, LDW, INFO )
11 CHARACTER UPLO
13 INTEGER INFO, KB, LDA, LDW, N, NB
15 INTEGER IPIV( * )
17 REAL A( LDA, * ), W( LDW, * )
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21 SLASYF computes a partial factorization of a real symmetric matrix A
22 using the Bunch-Kaufman diagonal pivoting method. The partial factor‐
23 ization has the form:
24 A = ( I U12 ) ( A11 0 ) ( I 0 ) if UPLO = 'U', or:
25 ( 0 U22 ) ( 0 D ) ( U12' U22' )
26 A = ( L11 0 ) ( D 0 ) ( L11' L21' ) if UPLO = 'L'
27 ( L21 I ) ( 0 A22 ) ( 0 I )
28 where the order of D is at most NB. The actual order is returned in the
29 argument KB, and is either NB or NB-1, or N if N <= NB. SLASYF is an
30 auxiliary routine called by SSYTRF. It uses blocked code (calling Level
31 3 BLAS) to update the submatrix A11 (if UPLO = 'U') or A22 (if UPLO =
32 'L').
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35 UPLO (input) CHARACTER*1
36 Specifies whether the upper or lower triangular part of the
37 symmetric matrix A is stored:
38 = 'U': Upper triangular
39 = 'L': Lower triangular
40 N (input) INTEGER
42 The order of the matrix A. N >= 0.
43 NB (input) INTEGER
45 The maximum number of columns of the matrix A that should be
46 factored. NB should be at least 2 to allow for 2-by-2 pivot
47 blocks.
48 KB (output) INTEGER
50 The number of columns of A that were actually factored. KB is
51 either NB-1 or NB, or N if N <= NB.
52 A (input/output) REAL array, dimension (LDA,N)
54 On entry, the symmetric matrix A. If UPLO = 'U', the leading
55 n-by-n upper triangular part of A contains the upper triangular
56 part of the matrix A, and the strictly lower triangular part of
57 A is not referenced. If UPLO = 'L', the leading n-by-n lower
58 triangular part of A contains the lower triangular part of the
59 matrix A, and the strictly upper triangular part of A is not
60 referenced. On exit, A contains details of the partial factor‐
61 ization.
62 LDA (input) INTEGER
64 The leading dimension of the array A. LDA >= max(1,N).
65 IPIV (output) INTEGER array, dimension (N)
67 Details of the interchanges and the block structure of D. If
68 UPLO = 'U', only the last KB elements of IPIV are set; if UPLO
69 = 'L', only the first KB elements are set. If IPIV(k) > 0,
70 then rows and columns k and IPIV(k) were interchanged and
71 D(k,k) is a 1-by-1 diagonal block. If UPLO = 'U' and IPIV(k) =
72 IPIV(k-1) < 0, then rows and columns k-1 and -IPIV(k) were
73 interchanged and D(k-1:k,k-1:k) is a 2-by-2 diagonal block. If
74 UPLO = 'L' and IPIV(k) = IPIV(k+1) < 0, then rows and columns
75 k+1 and -IPIV(k) were interchanged and D(k:k+1,k:k+1) is a
76 2-by-2 diagonal block.
77 W (workspace) REAL array, dimension (LDW,NB)
79 LDW (input) INTEGER
81 The leading dimension of the array W. LDW >= max(1,N).
82 INFO (output) INTEGER
84 = 0: successful exit
85 > 0: if INFO = k, D(k,k) is exactly zero. The factorization
86 has been completed, but the block diagonal matrix D is exactly
87 singular.
88 LAPACK routine (version 3.2) November 2008 SLASYF(1)