# NAG FL Interfacef07wkf (dtftri)

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## 1Purpose

f07wkf computes the inverse of a real triangular matrix stored in Rectangular Full Packed (RFP) format.

## 2Specification

Fortran Interface
 Subroutine f07wkf ( uplo, diag, n, ar, info)
 Integer, Intent (In) :: n Integer, Intent (Out) :: info Real (Kind=nag_wp), Intent (Inout) :: ar(n*(n+1)/2) Character (1), Intent (In) :: transr, uplo, diag
#include <nag.h>
 void f07wkf_ (const char *transr, const char *uplo, const char *diag, const Integer *n, double ar[], Integer *info, const Charlen length_transr, const Charlen length_uplo, const Charlen length_diag)
The routine may be called by the names f07wkf, nagf_lapacklin_dtftri or its LAPACK name dtftri.

## 3Description

f07wkf forms the inverse of a real triangular matrix $A$, stored using RFP format. The RFP storage format is described in Section 3.3.3 in the F07 Chapter Introduction. Note that the inverse of an upper (lower) triangular matrix is also upper (lower) triangular.

## 4References

Du Croz J J and Higham N J (1992) Stability of methods for matrix inversion IMA J. Numer. Anal. 12 1–19
Gustavson F G, Waśniewski J, Dongarra J J and Langou J (2010) Rectangular full packed format for Cholesky's algorithm: factorization, solution, and inversion ACM Trans. Math. Software 37, 2

## 5Arguments

1: $\mathbf{transr}$Character(1) Input
On entry: specifies whether the RFP representation of $A$ is normal or transposed.
${\mathbf{transr}}=\text{'N'}$
The matrix $A$ is stored in normal RFP format.
${\mathbf{transr}}=\text{'T'}$
The matrix $A$ is stored in transposed RFP format.
Constraint: ${\mathbf{transr}}=\text{'N'}$ or $\text{'T'}$.
2: $\mathbf{uplo}$Character(1) Input
On entry: specifies whether $A$ is upper or lower triangular.
${\mathbf{uplo}}=\text{'U'}$
$A$ is upper triangular.
${\mathbf{uplo}}=\text{'L'}$
$A$ is lower triangular.
Constraint: ${\mathbf{uplo}}=\text{'U'}$ or $\text{'L'}$.
3: $\mathbf{diag}$Character(1) Input
On entry: indicates whether $A$ is a nonunit or unit triangular matrix.
${\mathbf{diag}}=\text{'N'}$
$A$ is a nonunit triangular matrix.
${\mathbf{diag}}=\text{'U'}$
$A$ is a unit triangular matrix; the diagonal elements are not referenced and are assumed to be $1$.
Constraint: ${\mathbf{diag}}=\text{'N'}$ or $\text{'U'}$.
4: $\mathbf{n}$Integer Input
On entry: $n$, the order of the matrix $A$.
Constraint: ${\mathbf{n}}\ge 0$.
5: $\mathbf{ar}\left({\mathbf{n}}×\left({\mathbf{n}}+1\right)/2\right)$Real (Kind=nag_wp) array Input/Output
On entry: the upper or lower triangular part (as specified by uplo) of the $n×n$ symmetric matrix $A$, in either normal or transposed RFP format (as specified by transr). The storage format is described in detail in Section 3.3.3 in the F07 Chapter Introduction.
On exit: $A$ is overwritten by ${A}^{-1}$, in the same storage format as $A$.
6: $\mathbf{info}$Integer Output
On exit: ${\mathbf{info}}=0$ unless the routine detects an error (see Section 6).

## 6Error Indicators and Warnings

${\mathbf{info}}<0$
If ${\mathbf{info}}=-i$, argument $i$ had an illegal value. An explanatory message is output, and execution of the program is terminated.
${\mathbf{info}}>0$
Diagonal element $⟨\mathit{\text{value}}⟩$ of $A$ is exactly zero. $A$ is singular its inverse cannot be computed.

## 7Accuracy

The computed inverse $X$ satisfies
 $|XA-I|≤c(n)ε|X||A| ,$
where $c\left(n\right)$ is a modest linear function of $n$, and $\epsilon$ is the machine precision.
Note that a similar bound for $|AX-I|$ cannot be guaranteed, although it is almost always satisfied.
The computed inverse satisfies the forward error bound
 $|X-A-1|≤c(n)ε|A-1||A||X| .$
See Du Croz and Higham (1992).

## 8Parallelism and Performance

f07wkf makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

The total number of floating-point operations is approximately $\frac{1}{3}{n}^{3}$.
The complex analogue of this routine is f07wxf.

## 10Example

This example computes the inverse of the matrix $A$, where
 $A= ( 4.30 0.00 0.00 0.00 -3.96 -4.87 0.00 0.00 0.40 0.31 -8.02 0.00 -0.27 0.07 -5.95 0.12 )$
and is stored using RFP format.

### 10.1Program Text

Program Text (f07wkfe.f90)

### 10.2Program Data

Program Data (f07wkfe.d)

### 10.3Program Results

Program Results (f07wkfe.r)