---
name: "dgtt01.f"
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/dgtt01.f"
---

# dgtt01.f(3)

## NAME

TESTING/LIN/dgtt01.f

## SYNOPSIS

### Functions/Subroutines

subroutine **dgtt01** (n, dl, d, du, dlf, df, duf, du2, ipiv, work, ldwork, rwork, resid)\
 **DGTT01**

## Function/Subroutine Documentation

### subroutine dgtt01 (integer n, double precision, dimension( \* ) dl, double precision, dimension( \* ) d, double precision, dimension( \* ) du, double precision, dimension( \* ) dlf, double precision, dimension( \* ) df, double precision, dimension( \* ) duf, double precision, dimension( \* ) du2, integer, dimension( \* ) ipiv, double precision, dimension( ldwork, \* ) work, integer ldwork, double precision, dimension( \* ) rwork, double precision resid)

**DGTT01**

**Purpose:**

```
!>
!> DGTT01 reconstructs a tridiagonal matrix A from its LU factorization
!> and computes the residual
!>    norm(L*U - A) / ( norm(A) * EPS ),
!> where EPS is the machine epsilon.
!>
```

**Parameters**

*N*

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

*DL*

```
!>          DL is DOUBLE PRECISION array, dimension (N-1)
!>          The (n-1) sub-diagonal elements of A.
!>
```

*D*

```
!>          D is DOUBLE PRECISION array, dimension (N)
!>          The diagonal elements of A.
!>
```

*DU*

```
!>          DU is DOUBLE PRECISION array, dimension (N-1)
!>          The (n-1) super-diagonal elements of A.
!>
```

*DLF*

```
!>          DLF is DOUBLE PRECISION array, dimension (N-1)
!>          The (n-1) multipliers that define the matrix L from the
!>          LU factorization of A.
!>
```

*DF*

```
!>          DF is DOUBLE PRECISION array, dimension (N)
!>          The n diagonal elements of the upper triangular matrix U from
!>          the LU factorization of A.
!>
```

*DUF*

```
!>          DUF is DOUBLE PRECISION array, dimension (N-1)
!>          The (n-1) elements of the first super-diagonal of U.
!>
```

*DU2*

```
!>          DU2 is DOUBLE PRECISION array, dimension (N-2)
!>          The (n-2) elements of the second super-diagonal of U.
!>
```

*IPIV*

```
!>          IPIV is INTEGER array, dimension (N)
!>          The pivot indices; for 1 <= i <= n, row i of the matrix was
!>          interchanged with row IPIV(i).  IPIV(i) will always be either
!>          i or i+1; IPIV(i) = i indicates a row interchange was not
!>          required.
!>
```

*WORK*

```
!>          WORK is DOUBLE PRECISION array, dimension (LDWORK,N)
!>
```

*LDWORK*

```
!>          LDWORK is INTEGER
!>          The leading dimension of the array WORK.  LDWORK >= max(1,N).
!>
```

*RWORK*

```
!>          RWORK is DOUBLE PRECISION array, dimension (N)
!>
```

*RESID*

```
!>          RESID is DOUBLE PRECISION
!>          The scaled residual:  norm(L*U - A) / (norm(A) * EPS)
!>
```

**Author**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **132** of file **dgtt01.f**.

## Author

Generated automatically by Doxygen for LAPACK from the source code.