LAPACK  3.10.1
LAPACK: Linear Algebra PACKage

◆ zlantb()

double precision function zlantb ( character  NORM,
character  UPLO,
character  DIAG,
integer  N,
integer  K,
complex*16, dimension( ldab, * )  AB,
integer  LDAB,
double precision, dimension( * )  WORK 
)

ZLANTB returns the value of the 1-norm, or the Frobenius norm, or the infinity norm, or the element of largest absolute value of a triangular band matrix.

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Purpose:
 ZLANTB  returns the value of the one norm,  or the Frobenius norm, or
 the  infinity norm,  or the element of  largest absolute value  of an
 n by n triangular band matrix A,  with ( k + 1 ) diagonals.
Returns
ZLANTB
    ZLANTB = ( max(abs(A(i,j))), NORM = 'M' or 'm'
             (
             ( norm1(A),         NORM = '1', 'O' or 'o'
             (
             ( normI(A),         NORM = 'I' or 'i'
             (
             ( normF(A),         NORM = 'F', 'f', 'E' or 'e'

 where  norm1  denotes the  one norm of a matrix (maximum column sum),
 normI  denotes the  infinity norm  of a matrix  (maximum row sum) and
 normF  denotes the  Frobenius norm of a matrix (square root of sum of
 squares).  Note that  max(abs(A(i,j)))  is not a consistent matrix norm.
Parameters
[in]NORM
          NORM is CHARACTER*1
          Specifies the value to be returned in ZLANTB as described
          above.
[in]UPLO
          UPLO is CHARACTER*1
          Specifies whether the matrix A is upper or lower triangular.
          = 'U':  Upper triangular
          = 'L':  Lower triangular
[in]DIAG
          DIAG is CHARACTER*1
          Specifies whether or not the matrix A is unit triangular.
          = 'N':  Non-unit triangular
          = 'U':  Unit triangular
[in]N
          N is INTEGER
          The order of the matrix A.  N >= 0.  When N = 0, ZLANTB is
          set to zero.
[in]K
          K is INTEGER
          The number of super-diagonals of the matrix A if UPLO = 'U',
          or the number of sub-diagonals of the matrix A if UPLO = 'L'.
          K >= 0.
[in]AB
          AB is COMPLEX*16 array, dimension (LDAB,N)
          The upper or lower triangular band matrix A, stored in the
          first k+1 rows of AB.  The j-th column of A is stored
          in the j-th column of the array AB as follows:
          if UPLO = 'U', AB(k+1+i-j,j) = A(i,j) for max(1,j-k)<=i<=j;
          if UPLO = 'L', AB(1+i-j,j)   = A(i,j) for j<=i<=min(n,j+k).
          Note that when DIAG = 'U', the elements of the array AB
          corresponding to the diagonal elements of the matrix A are
          not referenced, but are assumed to be one.
[in]LDAB
          LDAB is INTEGER
          The leading dimension of the array AB.  LDAB >= K+1.
[out]WORK
          WORK is DOUBLE PRECISION array, dimension (MAX(1,LWORK)),
          where LWORK >= N when NORM = 'I'; otherwise, WORK is not
          referenced.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 139 of file zlantb.f.

141 *
142 * -- LAPACK auxiliary routine --
143 * -- LAPACK is a software package provided by Univ. of Tennessee, --
144 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
145 *
146 * .. Scalar Arguments ..
147  CHARACTER DIAG, NORM, UPLO
148  INTEGER K, LDAB, N
149 * ..
150 * .. Array Arguments ..
151  DOUBLE PRECISION WORK( * )
152  COMPLEX*16 AB( LDAB, * )
153 * ..
154 *
155 * =====================================================================
156 *
157 * .. Parameters ..
158  DOUBLE PRECISION ONE, ZERO
159  parameter( one = 1.0d+0, zero = 0.0d+0 )
160 * ..
161 * .. Local Scalars ..
162  LOGICAL UDIAG
163  INTEGER I, J, L
164  DOUBLE PRECISION SCALE, SUM, VALUE
165 * ..
166 * .. External Functions ..
167  LOGICAL LSAME, DISNAN
168  EXTERNAL lsame, disnan
169 * ..
170 * .. External Subroutines ..
171  EXTERNAL zlassq
172 * ..
173 * .. Intrinsic Functions ..
174  INTRINSIC abs, max, min, sqrt
175 * ..
176 * .. Executable Statements ..
177 *
178  IF( n.EQ.0 ) THEN
179  VALUE = zero
180  ELSE IF( lsame( norm, 'M' ) ) THEN
181 *
182 * Find max(abs(A(i,j))).
183 *
184  IF( lsame( diag, 'U' ) ) THEN
185  VALUE = one
186  IF( lsame( uplo, 'U' ) ) THEN
187  DO 20 j = 1, n
188  DO 10 i = max( k+2-j, 1 ), k
189  sum = abs( ab( i, j ) )
190  IF( VALUE .LT. sum .OR. disnan( sum ) ) VALUE = sum
191  10 CONTINUE
192  20 CONTINUE
193  ELSE
194  DO 40 j = 1, n
195  DO 30 i = 2, min( n+1-j, k+1 )
196  sum = abs( ab( i, j ) )
197  IF( VALUE .LT. sum .OR. disnan( sum ) ) VALUE = sum
198  30 CONTINUE
199  40 CONTINUE
200  END IF
201  ELSE
202  VALUE = zero
203  IF( lsame( uplo, 'U' ) ) THEN
204  DO 60 j = 1, n
205  DO 50 i = max( k+2-j, 1 ), k + 1
206  sum = abs( ab( i, j ) )
207  IF( VALUE .LT. sum .OR. disnan( sum ) ) VALUE = sum
208  50 CONTINUE
209  60 CONTINUE
210  ELSE
211  DO 80 j = 1, n
212  DO 70 i = 1, min( n+1-j, k+1 )
213  sum = abs( ab( i, j ) )
214  IF( VALUE .LT. sum .OR. disnan( sum ) ) VALUE = sum
215  70 CONTINUE
216  80 CONTINUE
217  END IF
218  END IF
219  ELSE IF( ( lsame( norm, 'O' ) ) .OR. ( norm.EQ.'1' ) ) THEN
220 *
221 * Find norm1(A).
222 *
223  VALUE = zero
224  udiag = lsame( diag, 'U' )
225  IF( lsame( uplo, 'U' ) ) THEN
226  DO 110 j = 1, n
227  IF( udiag ) THEN
228  sum = one
229  DO 90 i = max( k+2-j, 1 ), k
230  sum = sum + abs( ab( i, j ) )
231  90 CONTINUE
232  ELSE
233  sum = zero
234  DO 100 i = max( k+2-j, 1 ), k + 1
235  sum = sum + abs( ab( i, j ) )
236  100 CONTINUE
237  END IF
238  IF( VALUE .LT. sum .OR. disnan( sum ) ) VALUE = sum
239  110 CONTINUE
240  ELSE
241  DO 140 j = 1, n
242  IF( udiag ) THEN
243  sum = one
244  DO 120 i = 2, min( n+1-j, k+1 )
245  sum = sum + abs( ab( i, j ) )
246  120 CONTINUE
247  ELSE
248  sum = zero
249  DO 130 i = 1, min( n+1-j, k+1 )
250  sum = sum + abs( ab( i, j ) )
251  130 CONTINUE
252  END IF
253  IF( VALUE .LT. sum .OR. disnan( sum ) ) VALUE = sum
254  140 CONTINUE
255  END IF
256  ELSE IF( lsame( norm, 'I' ) ) THEN
257 *
258 * Find normI(A).
259 *
260  VALUE = zero
261  IF( lsame( uplo, 'U' ) ) THEN
262  IF( lsame( diag, 'U' ) ) THEN
263  DO 150 i = 1, n
264  work( i ) = one
265  150 CONTINUE
266  DO 170 j = 1, n
267  l = k + 1 - j
268  DO 160 i = max( 1, j-k ), j - 1
269  work( i ) = work( i ) + abs( ab( l+i, j ) )
270  160 CONTINUE
271  170 CONTINUE
272  ELSE
273  DO 180 i = 1, n
274  work( i ) = zero
275  180 CONTINUE
276  DO 200 j = 1, n
277  l = k + 1 - j
278  DO 190 i = max( 1, j-k ), j
279  work( i ) = work( i ) + abs( ab( l+i, j ) )
280  190 CONTINUE
281  200 CONTINUE
282  END IF
283  ELSE
284  IF( lsame( diag, 'U' ) ) THEN
285  DO 210 i = 1, n
286  work( i ) = one
287  210 CONTINUE
288  DO 230 j = 1, n
289  l = 1 - j
290  DO 220 i = j + 1, min( n, j+k )
291  work( i ) = work( i ) + abs( ab( l+i, j ) )
292  220 CONTINUE
293  230 CONTINUE
294  ELSE
295  DO 240 i = 1, n
296  work( i ) = zero
297  240 CONTINUE
298  DO 260 j = 1, n
299  l = 1 - j
300  DO 250 i = j, min( n, j+k )
301  work( i ) = work( i ) + abs( ab( l+i, j ) )
302  250 CONTINUE
303  260 CONTINUE
304  END IF
305  END IF
306  DO 270 i = 1, n
307  sum = work( i )
308  IF( VALUE .LT. sum .OR. disnan( sum ) ) VALUE = sum
309  270 CONTINUE
310  ELSE IF( ( lsame( norm, 'F' ) ) .OR. ( lsame( norm, 'E' ) ) ) THEN
311 *
312 * Find normF(A).
313 *
314  IF( lsame( uplo, 'U' ) ) THEN
315  IF( lsame( diag, 'U' ) ) THEN
316  scale = one
317  sum = n
318  IF( k.GT.0 ) THEN
319  DO 280 j = 2, n
320  CALL zlassq( min( j-1, k ),
321  $ ab( max( k+2-j, 1 ), j ), 1, scale,
322  $ sum )
323  280 CONTINUE
324  END IF
325  ELSE
326  scale = zero
327  sum = one
328  DO 290 j = 1, n
329  CALL zlassq( min( j, k+1 ), ab( max( k+2-j, 1 ), j ),
330  $ 1, scale, sum )
331  290 CONTINUE
332  END IF
333  ELSE
334  IF( lsame( diag, 'U' ) ) THEN
335  scale = one
336  sum = n
337  IF( k.GT.0 ) THEN
338  DO 300 j = 1, n - 1
339  CALL zlassq( min( n-j, k ), ab( 2, j ), 1, scale,
340  $ sum )
341  300 CONTINUE
342  END IF
343  ELSE
344  scale = zero
345  sum = one
346  DO 310 j = 1, n
347  CALL zlassq( min( n-j+1, k+1 ), ab( 1, j ), 1, scale,
348  $ sum )
349  310 CONTINUE
350  END IF
351  END IF
352  VALUE = scale*sqrt( sum )
353  END IF
354 *
355  zlantb = VALUE
356  RETURN
357 *
358 * End of ZLANTB
359 *
logical function disnan(DIN)
DISNAN tests input for NaN.
Definition: disnan.f:59
subroutine zlassq(n, x, incx, scl, sumsq)
ZLASSQ updates a sum of squares represented in scaled form.
Definition: zlassq.f90:137
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
double precision function zlantb(NORM, UPLO, DIAG, N, K, AB, LDAB, WORK)
ZLANTB returns the value of the 1-norm, or the Frobenius norm, or the infinity norm,...
Definition: zlantb.f:141
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