---
name: "ctbcon"
section: "3"
platform: "linux"
package: "lapack-doc"
version: "3.12.1-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/ctbcon.f"
---

# ctbcon(3)

Part of [ctbcon.f(3)](/ctbcon.f/3.md).

## NAME

SRC/ctbcon.f

## SYNOPSIS

### Functions/Subroutines

subroutine **ctbcon** (norm, uplo, diag, n, kd, ab, ldab, rcond, work, rwork, info)\
 **CTBCON**

## Function/Subroutine Documentation

### subroutine ctbcon (character norm, character uplo, character diag, integer n, integer kd, complex, dimension( ldab, \* ) ab, integer ldab, real rcond, complex, dimension( \* ) work, real, dimension( \* ) rwork, integer info)

**CTBCON**

**Purpose:**

```
!>
!> CTBCON estimates the reciprocal of the condition number of a
!> triangular band matrix A, in either the 1-norm or the infinity-norm.
!>
!> The norm of A is computed and an estimate is obtained for
!> norm(inv(A)), then the reciprocal of the condition number is
!> computed as
!>    RCOND = 1 / ( norm(A) * norm(inv(A)) ).
!>
```

**Parameters**

*NORM*

```
!>          NORM is CHARACTER*1
!>          Specifies whether the 1-norm condition number or the
!>          infinity-norm condition number is required:
!>          = '1' or 'O':  1-norm;
!>          = 'I':         Infinity-norm.
!>
```

*UPLO*

```
!>          UPLO is CHARACTER*1
!>          = 'U':  A is upper triangular;
!>          = 'L':  A is lower triangular.
!>
```

*DIAG*

```
!>          DIAG is CHARACTER*1
!>          = 'N':  A is non-unit triangular;
!>          = 'U':  A is unit triangular.
!>
```

*N*

```
!>          N is INTEGER
!>          The order of the matrix A.  N >= 0.
!>
```

*KD*

```
!>          KD is INTEGER
!>          The number of superdiagonals or subdiagonals of the
!>          triangular band matrix A.  KD >= 0.
!>
```

*AB*

```
!>          AB is COMPLEX array, dimension (LDAB,N)
!>          The upper or lower triangular band matrix A, stored in the
!>          first kd+1 rows of the array. The j-th column of A is stored
!>          in the j-th column of the array AB as follows:
!>          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
!>          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
!>          If DIAG = 'U', the diagonal elements of A are not referenced
!>          and are assumed to be 1.
!>
```

*LDAB*

```
!>          LDAB is INTEGER
!>          The leading dimension of the array AB.  LDAB >= KD+1.
!>
```

*RCOND*

```
!>          RCOND is REAL
!>          The reciprocal of the condition number of the matrix A,
!>          computed as RCOND = 1/(norm(A) * norm(inv(A))).
!>
```

*WORK*

```
!>          WORK is COMPLEX array, dimension (2*N)
!>
```

*RWORK*

```
!>          RWORK is REAL array, dimension (N)
!>
```

*INFO*

```
!>          INFO is INTEGER
!>          = 0:  successful exit
!>          < 0:  if INFO = -i, the i-th argument had an illegal value
!>
```

**Author**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **141** of file **ctbcon.f**.

## Author

Generated automatically by Doxygen for LAPACK from the source code.