1: SUBROUTINE ZPTTRS( UPLO, N, NRHS, D, E, B, LDB, INFO )
2: *
3: * -- LAPACK routine (version 3.2) --
4: * -- LAPACK is a software package provided by Univ. of Tennessee, --
5: * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
6: * November 2006
7: *
8: * .. Scalar Arguments ..
9: CHARACTER UPLO
10: INTEGER INFO, LDB, N, NRHS
11: * ..
12: * .. Array Arguments ..
13: DOUBLE PRECISION D( * )
14: COMPLEX*16 B( LDB, * ), E( * )
15: * ..
16: *
17: * Purpose
18: * =======
19: *
20: * ZPTTRS solves a tridiagonal system of the form
21: * A * X = B
22: * using the factorization A = U'*D*U or A = L*D*L' computed by ZPTTRF.
23: * D is a diagonal matrix specified in the vector D, U (or L) is a unit
24: * bidiagonal matrix whose superdiagonal (subdiagonal) is specified in
25: * the vector E, and X and B are N by NRHS matrices.
26: *
27: * Arguments
28: * =========
29: *
30: * UPLO (input) CHARACTER*1
31: * Specifies the form of the factorization and whether the
32: * vector E is the superdiagonal of the upper bidiagonal factor
33: * U or the subdiagonal of the lower bidiagonal factor L.
34: * = 'U': A = U'*D*U, E is the superdiagonal of U
35: * = 'L': A = L*D*L', E is the subdiagonal of L
36: *
37: * N (input) INTEGER
38: * The order of the tridiagonal matrix A. N >= 0.
39: *
40: * NRHS (input) INTEGER
41: * The number of right hand sides, i.e., the number of columns
42: * of the matrix B. NRHS >= 0.
43: *
44: * D (input) DOUBLE PRECISION array, dimension (N)
45: * The n diagonal elements of the diagonal matrix D from the
46: * factorization A = U'*D*U or A = L*D*L'.
47: *
48: * E (input) COMPLEX*16 array, dimension (N-1)
49: * If UPLO = 'U', the (n-1) superdiagonal elements of the unit
50: * bidiagonal factor U from the factorization A = U'*D*U.
51: * If UPLO = 'L', the (n-1) subdiagonal elements of the unit
52: * bidiagonal factor L from the factorization A = L*D*L'.
53: *
54: * B (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
55: * On entry, the right hand side vectors B for the system of
56: * linear equations.
57: * On exit, the solution vectors, X.
58: *
59: * LDB (input) INTEGER
60: * The leading dimension of the array B. LDB >= max(1,N).
61: *
62: * INFO (output) INTEGER
63: * = 0: successful exit
64: * < 0: if INFO = -k, the k-th argument had an illegal value
65: *
66: * =====================================================================
67: *
68: * .. Local Scalars ..
69: LOGICAL UPPER
70: INTEGER IUPLO, J, JB, NB
71: * ..
72: * .. External Functions ..
73: INTEGER ILAENV
74: EXTERNAL ILAENV
75: * ..
76: * .. External Subroutines ..
77: EXTERNAL XERBLA, ZPTTS2
78: * ..
79: * .. Intrinsic Functions ..
80: INTRINSIC MAX, MIN
81: * ..
82: * .. Executable Statements ..
83: *
84: * Test the input arguments.
85: *
86: INFO = 0
87: UPPER = ( UPLO.EQ.'U' .OR. UPLO.EQ.'u' )
88: IF( .NOT.UPPER .AND. .NOT.( UPLO.EQ.'L' .OR. UPLO.EQ.'l' ) ) THEN
89: INFO = -1
90: ELSE IF( N.LT.0 ) THEN
91: INFO = -2
92: ELSE IF( NRHS.LT.0 ) THEN
93: INFO = -3
94: ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
95: INFO = -7
96: END IF
97: IF( INFO.NE.0 ) THEN
98: CALL XERBLA( 'ZPTTRS', -INFO )
99: RETURN
100: END IF
101: *
102: * Quick return if possible
103: *
104: IF( N.EQ.0 .OR. NRHS.EQ.0 )
105: $ RETURN
106: *
107: * Determine the number of right-hand sides to solve at a time.
108: *
109: IF( NRHS.EQ.1 ) THEN
110: NB = 1
111: ELSE
112: NB = MAX( 1, ILAENV( 1, 'ZPTTRS', UPLO, N, NRHS, -1, -1 ) )
113: END IF
114: *
115: * Decode UPLO
116: *
117: IF( UPPER ) THEN
118: IUPLO = 1
119: ELSE
120: IUPLO = 0
121: END IF
122: *
123: IF( NB.GE.NRHS ) THEN
124: CALL ZPTTS2( IUPLO, N, NRHS, D, E, B, LDB )
125: ELSE
126: DO 10 J = 1, NRHS, NB
127: JB = MIN( NRHS-J+1, NB )
128: CALL ZPTTS2( IUPLO, N, JB, D, E, B( 1, J ), LDB )
129: 10 CONTINUE
130: END IF
131: *
132: RETURN
133: *
134: * End of ZPTTRS
135: *
136: END
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