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1 SUBROUTINE ZPTTRS( UPLO, N, NRHS, D, E, B, LDB, INFO ) |
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2 * |
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3 * -- LAPACK routine (version 3.1) -- |
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4 * Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. |
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5 * November 2006 |
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6 * |
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7 * .. Scalar Arguments .. |
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8 CHARACTER UPLO |
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9 INTEGER INFO, LDB, N, NRHS |
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10 * .. |
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11 * .. Array Arguments .. |
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12 DOUBLE PRECISION D( * ) |
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13 COMPLEX*16 B( LDB, * ), E( * ) |
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14 * .. |
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15 * |
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16 * Purpose |
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17 * ======= |
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18 * |
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19 * ZPTTRS solves a tridiagonal system of the form |
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20 * A * X = B |
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21 * using the factorization A = U'*D*U or A = L*D*L' computed by ZPTTRF. |
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22 * D is a diagonal matrix specified in the vector D, U (or L) is a unit |
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23 * bidiagonal matrix whose superdiagonal (subdiagonal) is specified in |
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24 * the vector E, and X and B are N by NRHS matrices. |
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25 * |
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26 * Arguments |
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27 * ========= |
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28 * |
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29 * UPLO (input) CHARACTER*1 |
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30 * Specifies the form of the factorization and whether the |
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31 * vector E is the superdiagonal of the upper bidiagonal factor |
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32 * U or the subdiagonal of the lower bidiagonal factor L. |
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33 * = 'U': A = U'*D*U, E is the superdiagonal of U |
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34 * = 'L': A = L*D*L', E is the subdiagonal of L |
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35 * |
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36 * N (input) INTEGER |
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37 * The order of the tridiagonal matrix A. N >= 0. |
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38 * |
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39 * NRHS (input) INTEGER |
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40 * The number of right hand sides, i.e., the number of columns |
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41 * of the matrix B. NRHS >= 0. |
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42 * |
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43 * D (input) DOUBLE PRECISION array, dimension (N) |
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44 * The n diagonal elements of the diagonal matrix D from the |
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45 * factorization A = U'*D*U or A = L*D*L'. |
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46 * |
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47 * E (input) COMPLEX*16 array, dimension (N-1) |
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48 * If UPLO = 'U', the (n-1) superdiagonal elements of the unit |
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49 * bidiagonal factor U from the factorization A = U'*D*U. |
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50 * If UPLO = 'L', the (n-1) subdiagonal elements of the unit |
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51 * bidiagonal factor L from the factorization A = L*D*L'. |
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52 * |
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53 * B (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS) |
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54 * On entry, the right hand side vectors B for the system of |
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55 * linear equations. |
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56 * On exit, the solution vectors, X. |
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57 * |
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58 * LDB (input) INTEGER |
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59 * The leading dimension of the array B. LDB >= max(1,N). |
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60 * |
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61 * INFO (output) INTEGER |
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62 * = 0: successful exit |
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63 * < 0: if INFO = -k, the k-th argument had an illegal value |
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64 * |
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65 * ===================================================================== |
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66 * |
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67 * .. Local Scalars .. |
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68 LOGICAL UPPER |
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69 INTEGER IUPLO, J, JB, NB |
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70 * .. |
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71 * .. External Functions .. |
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72 INTEGER ILAENV |
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73 EXTERNAL ILAENV |
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74 * .. |
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75 * .. External Subroutines .. |
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76 EXTERNAL XERBLA, ZPTTS2 |
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77 * .. |
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78 * .. Intrinsic Functions .. |
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79 INTRINSIC MAX, MIN |
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80 * .. |
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81 * .. Executable Statements .. |
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82 * |
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83 * Test the input arguments. |
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84 * |
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85 INFO = 0 |
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86 UPPER = ( UPLO.EQ.'U' .OR. UPLO.EQ.'u' ) |
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87 IF( .NOT.UPPER .AND. .NOT.( UPLO.EQ.'L' .OR. UPLO.EQ.'l' ) ) THEN |
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88 INFO = -1 |
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89 ELSE IF( N.LT.0 ) THEN |
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90 INFO = -2 |
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91 ELSE IF( NRHS.LT.0 ) THEN |
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92 INFO = -3 |
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93 ELSE IF( LDB.LT.MAX( 1, N ) ) THEN |
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94 INFO = -7 |
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95 END IF |
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96 IF( INFO.NE.0 ) THEN |
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97 CALL XERBLA( 'ZPTTRS', -INFO ) |
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98 RETURN |
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99 END IF |
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100 * |
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101 * Quick return if possible |
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102 * |
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103 IF( N.EQ.0 .OR. NRHS.EQ.0 ) |
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104 $ RETURN |
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105 * |
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106 * Determine the number of right-hand sides to solve at a time. |
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107 * |
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108 IF( NRHS.EQ.1 ) THEN |
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109 NB = 1 |
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110 ELSE |
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111 NB = MAX( 1, ILAENV( 1, 'ZPTTRS', UPLO, N, NRHS, -1, -1 ) ) |
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112 END IF |
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113 * |
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114 * Decode UPLO |
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115 * |
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116 IF( UPPER ) THEN |
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117 IUPLO = 1 |
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118 ELSE |
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119 IUPLO = 0 |
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120 END IF |
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121 * |
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122 IF( NB.GE.NRHS ) THEN |
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123 CALL ZPTTS2( IUPLO, N, NRHS, D, E, B, LDB ) |
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124 ELSE |
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125 DO 10 J = 1, NRHS, NB |
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126 JB = MIN( NRHS-J+1, NB ) |
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127 CALL ZPTTS2( IUPLO, N, JB, D, E, B( 1, J ), LDB ) |
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128 10 CONTINUE |
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129 END IF |
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130 * |
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131 RETURN |
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132 * |
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133 * End of ZPTTRS |
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134 * |
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135 END |
