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LAPACK 3.3.0
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LAPACK 3.3.0
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00001 SUBROUTINE SGBTRS( TRANS, N, KL, KU, NRHS, AB, LDAB, IPIV, B, LDB, 00002 $ INFO ) 00003 * 00004 * -- LAPACK routine (version 3.2) -- 00005 * -- LAPACK is a software package provided by Univ. of Tennessee, -- 00006 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- 00007 * November 2006 00008 * 00009 * .. Scalar Arguments .. 00010 CHARACTER TRANS 00011 INTEGER INFO, KL, KU, LDAB, LDB, N, NRHS 00012 * .. 00013 * .. Array Arguments .. 00014 INTEGER IPIV( * ) 00015 REAL AB( LDAB, * ), B( LDB, * ) 00016 * .. 00017 * 00018 * Purpose 00019 * ======= 00020 * 00021 * SGBTRS solves a system of linear equations 00022 * A * X = B or A' * X = B 00023 * with a general band matrix A using the LU factorization computed 00024 * by SGBTRF. 00025 * 00026 * Arguments 00027 * ========= 00028 * 00029 * TRANS (input) CHARACTER*1 00030 * Specifies the form of the system of equations. 00031 * = 'N': A * X = B (No transpose) 00032 * = 'T': A'* X = B (Transpose) 00033 * = 'C': A'* X = B (Conjugate transpose = Transpose) 00034 * 00035 * N (input) INTEGER 00036 * The order of the matrix A. N >= 0. 00037 * 00038 * KL (input) INTEGER 00039 * The number of subdiagonals within the band of A. KL >= 0. 00040 * 00041 * KU (input) INTEGER 00042 * The number of superdiagonals within the band of A. KU >= 0. 00043 * 00044 * NRHS (input) INTEGER 00045 * The number of right hand sides, i.e., the number of columns 00046 * of the matrix B. NRHS >= 0. 00047 * 00048 * AB (input) REAL array, dimension (LDAB,N) 00049 * Details of the LU factorization of the band matrix A, as 00050 * computed by SGBTRF. U is stored as an upper triangular band 00051 * matrix with KL+KU superdiagonals in rows 1 to KL+KU+1, and 00052 * the multipliers used during the factorization are stored in 00053 * rows KL+KU+2 to 2*KL+KU+1. 00054 * 00055 * LDAB (input) INTEGER 00056 * The leading dimension of the array AB. LDAB >= 2*KL+KU+1. 00057 * 00058 * IPIV (input) INTEGER array, dimension (N) 00059 * The pivot indices; for 1 <= i <= N, row i of the matrix was 00060 * interchanged with row IPIV(i). 00061 * 00062 * B (input/output) REAL array, dimension (LDB,NRHS) 00063 * On entry, the right hand side matrix B. 00064 * On exit, the solution matrix X. 00065 * 00066 * LDB (input) INTEGER 00067 * The leading dimension of the array B. LDB >= max(1,N). 00068 * 00069 * INFO (output) INTEGER 00070 * = 0: successful exit 00071 * < 0: if INFO = -i, the i-th argument had an illegal value 00072 * 00073 * ===================================================================== 00074 * 00075 * .. Parameters .. 00076 REAL ONE 00077 PARAMETER ( ONE = 1.0E+0 ) 00078 * .. 00079 * .. Local Scalars .. 00080 LOGICAL LNOTI, NOTRAN 00081 INTEGER I, J, KD, L, LM 00082 * .. 00083 * .. External Functions .. 00084 LOGICAL LSAME 00085 EXTERNAL LSAME 00086 * .. 00087 * .. External Subroutines .. 00088 EXTERNAL SGEMV, SGER, SSWAP, STBSV, XERBLA 00089 * .. 00090 * .. Intrinsic Functions .. 00091 INTRINSIC MAX, MIN 00092 * .. 00093 * .. Executable Statements .. 00094 * 00095 * Test the input parameters. 00096 * 00097 INFO = 0 00098 NOTRAN = LSAME( TRANS, 'N' ) 00099 IF( .NOT.NOTRAN .AND. .NOT.LSAME( TRANS, 'T' ) .AND. .NOT. 00100 $ LSAME( TRANS, 'C' ) ) THEN 00101 INFO = -1 00102 ELSE IF( N.LT.0 ) THEN 00103 INFO = -2 00104 ELSE IF( KL.LT.0 ) THEN 00105 INFO = -3 00106 ELSE IF( KU.LT.0 ) THEN 00107 INFO = -4 00108 ELSE IF( NRHS.LT.0 ) THEN 00109 INFO = -5 00110 ELSE IF( LDAB.LT.( 2*KL+KU+1 ) ) THEN 00111 INFO = -7 00112 ELSE IF( LDB.LT.MAX( 1, N ) ) THEN 00113 INFO = -10 00114 END IF 00115 IF( INFO.NE.0 ) THEN 00116 CALL XERBLA( 'SGBTRS', -INFO ) 00117 RETURN 00118 END IF 00119 * 00120 * Quick return if possible 00121 * 00122 IF( N.EQ.0 .OR. NRHS.EQ.0 ) 00123 $ RETURN 00124 * 00125 KD = KU + KL + 1 00126 LNOTI = KL.GT.0 00127 * 00128 IF( NOTRAN ) THEN 00129 * 00130 * Solve A*X = B. 00131 * 00132 * Solve L*X = B, overwriting B with X. 00133 * 00134 * L is represented as a product of permutations and unit lower 00135 * triangular matrices L = P(1) * L(1) * ... * P(n-1) * L(n-1), 00136 * where each transformation L(i) is a rank-one modification of 00137 * the identity matrix. 00138 * 00139 IF( LNOTI ) THEN 00140 DO 10 J = 1, N - 1 00141 LM = MIN( KL, N-J ) 00142 L = IPIV( J ) 00143 IF( L.NE.J ) 00144 $ CALL SSWAP( NRHS, B( L, 1 ), LDB, B( J, 1 ), LDB ) 00145 CALL SGER( LM, NRHS, -ONE, AB( KD+1, J ), 1, B( J, 1 ), 00146 $ LDB, B( J+1, 1 ), LDB ) 00147 10 CONTINUE 00148 END IF 00149 * 00150 DO 20 I = 1, NRHS 00151 * 00152 * Solve U*X = B, overwriting B with X. 00153 * 00154 CALL STBSV( 'Upper', 'No transpose', 'Non-unit', N, KL+KU, 00155 $ AB, LDAB, B( 1, I ), 1 ) 00156 20 CONTINUE 00157 * 00158 ELSE 00159 * 00160 * Solve A'*X = B. 00161 * 00162 DO 30 I = 1, NRHS 00163 * 00164 * Solve U'*X = B, overwriting B with X. 00165 * 00166 CALL STBSV( 'Upper', 'Transpose', 'Non-unit', N, KL+KU, AB, 00167 $ LDAB, B( 1, I ), 1 ) 00168 30 CONTINUE 00169 * 00170 * Solve L'*X = B, overwriting B with X. 00171 * 00172 IF( LNOTI ) THEN 00173 DO 40 J = N - 1, 1, -1 00174 LM = MIN( KL, N-J ) 00175 CALL SGEMV( 'Transpose', LM, NRHS, -ONE, B( J+1, 1 ), 00176 $ LDB, AB( KD+1, J ), 1, ONE, B( J, 1 ), LDB ) 00177 L = IPIV( J ) 00178 IF( L.NE.J ) 00179 $ CALL SSWAP( NRHS, B( L, 1 ), LDB, B( J, 1 ), LDB ) 00180 40 CONTINUE 00181 END IF 00182 END IF 00183 RETURN 00184 * 00185 * End of SGBTRS 00186 * 00187 END
1.7.3 
