1: SUBROUTINE DPTTRS( 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: INTEGER INFO, LDB, N, NRHS
10: * ..
11: * .. Array Arguments ..
12: DOUBLE PRECISION B( LDB, * ), D( * ), E( * )
13: * ..
14: *
15: * Purpose
16: * =======
17: *
18: * DPTTRS solves a tridiagonal system of the form
19: * A * X = B
20: * using the L*D*L' factorization of A computed by DPTTRF. D is a
21: * diagonal matrix specified in the vector D, L is a unit bidiagonal
22: * matrix whose subdiagonal is specified in the vector E, and X and B
23: * are N by NRHS matrices.
24: *
25: * Arguments
26: * =========
27: *
28: * N (input) INTEGER
29: * The order of the tridiagonal matrix A. N >= 0.
30: *
31: * NRHS (input) INTEGER
32: * The number of right hand sides, i.e., the number of columns
33: * of the matrix B. NRHS >= 0.
34: *
35: * D (input) DOUBLE PRECISION array, dimension (N)
36: * The n diagonal elements of the diagonal matrix D from the
37: * L*D*L' factorization of A.
38: *
39: * E (input) DOUBLE PRECISION array, dimension (N-1)
40: * The (n-1) subdiagonal elements of the unit bidiagonal factor
41: * L from the L*D*L' factorization of A. E can also be regarded
42: * as the superdiagonal of the unit bidiagonal factor U from the
43: * factorization A = U'*D*U.
44: *
45: * B (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
46: * On entry, the right hand side vectors B for the system of
47: * linear equations.
48: * On exit, the solution vectors, X.
49: *
50: * LDB (input) INTEGER
51: * The leading dimension of the array B. LDB >= max(1,N).
52: *
53: * INFO (output) INTEGER
54: * = 0: successful exit
55: * < 0: if INFO = -k, the k-th argument had an illegal value
56: *
57: * =====================================================================
58: *
59: * .. Local Scalars ..
60: INTEGER J, JB, NB
61: * ..
62: * .. External Functions ..
63: INTEGER ILAENV
64: EXTERNAL ILAENV
65: * ..
66: * .. External Subroutines ..
67: EXTERNAL DPTTS2, XERBLA
68: * ..
69: * .. Intrinsic Functions ..
70: INTRINSIC MAX, MIN
71: * ..
72: * .. Executable Statements ..
73: *
74: * Test the input arguments.
75: *
76: INFO = 0
77: IF( N.LT.0 ) THEN
78: INFO = -1
79: ELSE IF( NRHS.LT.0 ) THEN
80: INFO = -2
81: ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
82: INFO = -6
83: END IF
84: IF( INFO.NE.0 ) THEN
85: CALL XERBLA( 'DPTTRS', -INFO )
86: RETURN
87: END IF
88: *
89: * Quick return if possible
90: *
91: IF( N.EQ.0 .OR. NRHS.EQ.0 )
92: $ RETURN
93: *
94: * Determine the number of right-hand sides to solve at a time.
95: *
96: IF( NRHS.EQ.1 ) THEN
97: NB = 1
98: ELSE
99: NB = MAX( 1, ILAENV( 1, 'DPTTRS', ' ', N, NRHS, -1, -1 ) )
100: END IF
101: *
102: IF( NB.GE.NRHS ) THEN
103: CALL DPTTS2( N, NRHS, D, E, B, LDB )
104: ELSE
105: DO 10 J = 1, NRHS, NB
106: JB = MIN( NRHS-J+1, NB )
107: CALL DPTTS2( N, JB, D, E, B( 1, J ), LDB )
108: 10 CONTINUE
109: END IF
110: *
111: RETURN
112: *
113: * End of DPTTRS
114: *
115: END
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