1SLATBS(1) LAPACK auxiliary routine (version 3.1) SLATBS(1)
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6 SLATBS - one of the triangular systems A *x = s*b or A'*x = s*b with
7 scaling to prevent overflow, where A is an upper or lower triangular
8 band matrix
9
11 SUBROUTINE SLATBS( UPLO, TRANS, DIAG, NORMIN, N, KD, AB, LDAB, X,
12 SCALE, CNORM, INFO )
13 CHARACTER DIAG, NORMIN, TRANS, UPLO
15 INTEGER INFO, KD, LDAB, N
17 REAL SCALE
19 REAL AB( LDAB, * ), CNORM( * ), X( * )
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23 SLATBS solves one of the triangular systems are n-element vectors, and
24 s is a scaling factor, usually less than or equal to 1, chosen so that
25 the components of x will be less than the overflow threshold. If the
26 unscaled problem will not cause overflow, the Level 2 BLAS routine
27 STBSV is called. If the matrix A is singular (A(j,j) = 0 for some j),
28 then s is set to 0 and a non-trivial solution to A*x = 0 is returned.
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32 UPLO (input) CHARACTER*1
33 Specifies whether the matrix A is upper or lower triangular. =
34 'U': Upper triangular
35 = 'L': Lower triangular
36 TRANS (input) CHARACTER*1
38 Specifies the operation applied to A. = 'N': Solve A * x =
39 s*b (No transpose)
40 = 'T': Solve A'* x = s*b (Transpose)
41 = 'C': Solve A'* x = s*b (Conjugate transpose = Transpose)
42 DIAG (input) CHARACTER*1
44 Specifies whether or not the matrix A is unit triangular. =
45 'N': Non-unit triangular
46 = 'U': Unit triangular
47 NORMIN (input) CHARACTER*1
49 Specifies whether CNORM has been set or not. = 'Y': CNORM
50 contains the column norms on entry
51 = 'N': CNORM is not set on entry. On exit, the norms will be
52 computed and stored in CNORM.
53 N (input) INTEGER
55 The order of the matrix A. N >= 0.
56 KD (input) INTEGER
58 The number of subdiagonals or superdiagonals in the triangular
59 matrix A. KD >= 0.
60 AB (input) REAL array, dimension (LDAB,N)
62 The upper or lower triangular band matrix A, stored in the
63 first KD+1 rows of the array. The j-th column of A is stored in
64 the j-th column of the array AB as follows: if UPLO = 'U',
65 AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j; if UPLO = 'L',
66 AB(1+i-j,j) = A(i,j) for j<=i<=min(n,j+kd).
67 LDAB (input) INTEGER
69 The leading dimension of the array AB. LDAB >= KD+1.
70 X (input/output) REAL array, dimension (N)
72 On entry, the right hand side b of the triangular system. On
73 exit, X is overwritten by the solution vector x.
74 SCALE (output) REAL
76 The scaling factor s for the triangular system A * x = s*b or
77 A'* x = s*b. If SCALE = 0, the matrix A is singular or badly
78 scaled, and the vector x is an exact or approximate solution to
79 A*x = 0.
80 CNORM (input or output) REAL array, dimension (N)
82 If NORMIN = 'Y', CNORM is an input argument and CNORM(j) con‐
84 tains the norm of the off-diagonal part of the j-th column of
85 A. If TRANS = 'N', CNORM(j) must be greater than or equal to
86 the infinity-norm, and if TRANS = 'T' or 'C', CNORM(j) must be
87 greater than or equal to the 1-norm.
88 If NORMIN = 'N', CNORM is an output argument and CNORM(j)
90 returns the 1-norm of the offdiagonal part of the j-th column
91 of A.
92 INFO (output) INTEGER
94 = 0: successful exit
95 < 0: if INFO = -k, the k-th argument had an illegal value
96
98 A rough bound on x is computed; if that is less than overflow, STBSV is
99 called, otherwise, specific code is used which checks for possible
100 overflow or divide-by-zero at every operation.
101 A columnwise scheme is used for solving A*x = b. The basic algorithm
103 if A is lower triangular is
104 x[1:n] := b[1:n]
106 for j = 1, ..., n
107 x(j) := x(j) / A(j,j)
108 x[j+1:n] := x[j+1:n] - x(j) * A[j+1:n,j]
109 end
110 Define bounds on the components of x after j iterations of the loop:
112 M(j) = bound on x[1:j]
113 G(j) = bound on x[j+1:n]
114 Initially, let M(0) = 0 and G(0) = max{x(i), i=1,...,n}.
115 Then for iteration j+1 we have
117 M(j+1) <= G(j) / | A(j+1,j+1) |
118 G(j+1) <= G(j) + M(j+1) * | A[j+2:n,j+1] |
119 <= G(j) ( 1 + CNORM(j+1) / | A(j+1,j+1) | )
120 where CNORM(j+1) is greater than or equal to the infinity-norm of col‐
122 umn j+1 of A, not counting the diagonal. Hence
123 G(j) <= G(0) product ( 1 + CNORM(i) / | A(i,i) | )
125 1<=i<=j
126 and
127 |x(j)| <= ( G(0) / |A(j,j)| ) product ( 1 + CNORM(i) / |A(i,i)| )
129 1<=i< j
130 Since |x(j)| <= M(j), we use the Level 2 BLAS routine STBSV if the
132 reciprocal of the largest M(j), j=1,..,n, is larger than
133 max(underflow, 1/overflow).
134 The bound on x(j) is also used to determine when a step in the column‐
136 wise method can be performed without fear of overflow. If the computed
137 bound is greater than a large constant, x is scaled to prevent over‐
138 flow, but if the bound overflows, x is set to 0, x(j) to 1, and scale
139 to 0, and a non-trivial solution to A*x = 0 is found.
140 Similarly, a row-wise scheme is used to solve A'*x = b. The basic
142 algorithm for A upper triangular is
143 for j = 1, ..., n
145 x(j) := ( b(j) - A[1:j-1,j]' * x[1:j-1] ) / A(j,j)
146 end
147 We simultaneously compute two bounds
149 G(j) = bound on ( b(i) - A[1:i-1,i]' * x[1:i-1] ), 1<=i<=j
150 M(j) = bound on x(i), 1<=i<=j
151 The initial values are G(0) = 0, M(0) = max{b(i), i=1,..,n}, and we add
153 the constraint G(j) >= G(j-1) and M(j) >= M(j-1) for j >= 1. Then the
154 bound on x(j) is
155 M(j) <= M(j-1) * ( 1 + CNORM(j) ) / | A(j,j) |
157 <= M(0) * product ( ( 1 + CNORM(i) ) / |A(i,i)| )
159 1<=i<=j
160 and we can safely call STBSV if 1/M(n) and 1/G(n) are both greater than
162 max(underflow, 1/overflow).
163 LAPACK auxiliary routine (versionNo3v.e1m)ber 2006 SLATBS(1)