 LAPACK 3.11.0 LAPACK: Linear Algebra PACKage
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## ◆ clangt()

 real function clangt ( character NORM, integer N, complex, dimension( * ) DL, complex, dimension( * ) D, complex, dimension( * ) DU )

CLANGT returns the value of the 1-norm, Frobenius norm, infinity-norm, or the largest absolute value of any element of a general tridiagonal matrix.

Purpose:
``` CLANGT  returns the value of the one norm,  or the Frobenius norm, or
the  infinity norm,  or the  element of  largest absolute value  of a
complex tridiagonal matrix A.```
Returns
CLANGT
```    CLANGT = ( 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 CLANGT as described above.``` [in] N ``` N is INTEGER The order of the matrix A. N >= 0. When N = 0, CLANGT is set to zero.``` [in] DL ``` DL is COMPLEX array, dimension (N-1) The (n-1) sub-diagonal elements of A.``` [in] D ``` D is COMPLEX array, dimension (N) The diagonal elements of A.``` [in] DU ``` DU is COMPLEX array, dimension (N-1) The (n-1) super-diagonal elements of A.```

Definition at line 105 of file clangt.f.

106*
107* -- LAPACK auxiliary routine --
108* -- LAPACK is a software package provided by Univ. of Tennessee, --
109* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
110*
111* .. Scalar Arguments ..
112 CHARACTER NORM
113 INTEGER N
114* ..
115* .. Array Arguments ..
116 COMPLEX D( * ), DL( * ), DU( * )
117* ..
118*
119* =====================================================================
120*
121* .. Parameters ..
122 REAL ONE, ZERO
123 parameter( one = 1.0e+0, zero = 0.0e+0 )
124* ..
125* .. Local Scalars ..
126 INTEGER I
127 REAL ANORM, SCALE, SUM, TEMP
128* ..
129* .. External Functions ..
130 LOGICAL LSAME, SISNAN
131 EXTERNAL lsame, sisnan
132* ..
133* .. External Subroutines ..
134 EXTERNAL classq
135* ..
136* .. Intrinsic Functions ..
137 INTRINSIC abs, sqrt
138* ..
139* .. Executable Statements ..
140*
141 IF( n.LE.0 ) THEN
142 anorm = zero
143 ELSE IF( lsame( norm, 'M' ) ) THEN
144*
145* Find max(abs(A(i,j))).
146*
147 anorm = abs( d( n ) )
148 DO 10 i = 1, n - 1
149 IF( anorm.LT.abs( dl( i ) ) .OR. sisnan( abs( dl( i ) ) ) )
150 \$ anorm = abs(dl(i))
151 IF( anorm.LT.abs( d( i ) ) .OR. sisnan( abs( d( i ) ) ) )
152 \$ anorm = abs(d(i))
153 IF( anorm.LT.abs( du( i ) ) .OR. sisnan(abs( du( i ) ) ) )
154 \$ anorm = abs(du(i))
155 10 CONTINUE
156 ELSE IF( lsame( norm, 'O' ) .OR. norm.EQ.'1' ) THEN
157*
158* Find norm1(A).
159*
160 IF( n.EQ.1 ) THEN
161 anorm = abs( d( 1 ) )
162 ELSE
163 anorm = abs( d( 1 ) )+abs( dl( 1 ) )
164 temp = abs( d( n ) )+abs( du( n-1 ) )
165 IF( anorm .LT. temp .OR. sisnan( temp ) ) anorm = temp
166 DO 20 i = 2, n - 1
167 temp = abs( d( i ) )+abs( dl( i ) )+abs( du( i-1 ) )
168 IF( anorm .LT. temp .OR. sisnan( temp ) ) anorm = temp
169 20 CONTINUE
170 END IF
171 ELSE IF( lsame( norm, 'I' ) ) THEN
172*
173* Find normI(A).
174*
175 IF( n.EQ.1 ) THEN
176 anorm = abs( d( 1 ) )
177 ELSE
178 anorm = abs( d( 1 ) )+abs( du( 1 ) )
179 temp = abs( d( n ) )+abs( dl( n-1 ) )
180 IF( anorm .LT. temp .OR. sisnan( temp ) ) anorm = temp
181 DO 30 i = 2, n - 1
182 temp = abs( d( i ) )+abs( du( i ) )+abs( dl( i-1 ) )
183 IF( anorm .LT. temp .OR. sisnan( temp ) ) anorm = temp
184 30 CONTINUE
185 END IF
186 ELSE IF( ( lsame( norm, 'F' ) ) .OR. ( lsame( norm, 'E' ) ) ) THEN
187*
188* Find normF(A).
189*
190 scale = zero
191 sum = one
192 CALL classq( n, d, 1, scale, sum )
193 IF( n.GT.1 ) THEN
194 CALL classq( n-1, dl, 1, scale, sum )
195 CALL classq( n-1, du, 1, scale, sum )
196 END IF
197 anorm = scale*sqrt( sum )
198 END IF
199*
200 clangt = anorm
201 RETURN
202*
203* End of CLANGT
204*
subroutine classq(n, x, incx, scl, sumsq)
CLASSQ updates a sum of squares represented in scaled form.
Definition: classq.f90:137
logical function sisnan(SIN)
SISNAN tests input for NaN.
Definition: sisnan.f:59
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
real function clangt(NORM, N, DL, D, DU)
CLANGT returns the value of the 1-norm, Frobenius norm, infinity-norm, or the largest absolute value ...
Definition: clangt.f:106
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