# NAG CL Interfacec05rdc (sys_​deriv_​rcomm)

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## 1Purpose

c05rdc is a comprehensive reverse communication function that finds a solution of a system of nonlinear equations by a modification of the Powell hybrid method. You must provide the Jacobian.

## 2Specification

 #include
 void c05rdc (Integer *irevcm, Integer n, double x[], double fvec[], double fjac[], double xtol, Nag_ScaleType scale_mode, double diag[], double factor, double r[], double qtf[], Integer iwsav[], double rwsav[], NagError *fail)
The function may be called by the names: c05rdc, nag_roots_sys_deriv_rcomm or nag_zero_nonlin_eqns_deriv_rcomm.

## 3Description

The system of equations is defined as:
 $fi (x1,x2,…,xn) = 0 , i= 1, 2, …, n .$
c05rdc is based on the MINPACK routine HYBRJ (see Moré et al. (1980)). It chooses the correction at each step as a convex combination of the Newton and scaled gradient directions. The Jacobian is updated by the rank-1 method of Broyden. For more details see Powell (1970).

## 4References

Moré J J, Garbow B S and Hillstrom K E (1980) User guide for MINPACK-1 Technical Report ANL-80-74 Argonne National Laboratory
Powell M J D (1970) A hybrid method for nonlinear algebraic equations Numerical Methods for Nonlinear Algebraic Equations (ed P Rabinowitz) Gordon and Breach

## 5Arguments

Note: this function uses reverse communication. Its use involves an initial entry, intermediate exits and re-entries, and a final exit, as indicated by the argument irevcm. Between intermediate exits and re-entries, all arguments other than fvec and fjac must remain unchanged.
1: $\mathbf{irevcm}$Integer * Input/Output
On initial entry: must have the value $0$.
On intermediate exit: specifies what action you must take before re-entering c05rdc with irevcm unchanged. The value of irevcm should be interpreted as follows:
${\mathbf{irevcm}}=1$
Indicates the start of a new iteration. No action is required by you, but x and fvec are available for printing.
${\mathbf{irevcm}}=2$
Indicates that before re-entry to c05rdc, fvec must contain the function values ${f}_{i}\left(x\right)$.
${\mathbf{irevcm}}=3$
Indicates that before re-entry to c05rdc, ${\mathbf{fjac}}\left[\left(\mathit{j}-1\right)×{\mathbf{n}}+\mathit{i}-1\right]$ must contain the value of $\frac{\partial {f}_{\mathit{i}}}{\partial {x}_{\mathit{j}}}$ at the point $x$, for $\mathit{i}=1,2,\dots ,n$ and $\mathit{j}=1,2,\dots ,n$.
On final exit: ${\mathbf{irevcm}}=0$ and the algorithm has terminated.
Constraint: ${\mathbf{irevcm}}=0$, $1$, $2$ or $3$.
Note: any values you return to c05rdc as part of the reverse communication procedure should not include floating-point NaN (Not a Number) or infinity values, since these are not handled by c05rdc. If your code inadvertently does return any NaNs or infinities, c05rdc is likely to produce unexpected results.
2: $\mathbf{n}$Integer Input
On entry: $n$, the number of equations.
Constraint: ${\mathbf{n}}>0$.
3: $\mathbf{x}\left[{\mathbf{n}}\right]$double Input/Output
On initial entry: an initial guess at the solution vector.
On intermediate exit: contains the current point.
On final exit: the final estimate of the solution vector.
4: $\mathbf{fvec}\left[{\mathbf{n}}\right]$double Input/Output
On initial entry: need not be set.
On intermediate re-entry: if ${\mathbf{irevcm}}\ne 2$, fvec must not be changed.
If ${\mathbf{irevcm}}=2$, fvec must be set to the values of the functions computed at the current point x.
On final exit: the function values at the final point, x.
5: $\mathbf{fjac}\left[{\mathbf{n}}×{\mathbf{n}}\right]$double Input/Output
Note: the $\left(i,j\right)$th element of the matrix is stored in ${\mathbf{fjac}}\left[\left(j-1\right)×{\mathbf{n}}+i-1\right]$.
On initial entry: need not be set.
On intermediate re-entry: if ${\mathbf{irevcm}}\ne 3$, fjac must not be changed.
If ${\mathbf{irevcm}}=3$, ${\mathbf{fjac}}\left[\left(\mathit{j}-1\right)×{\mathbf{n}}+\mathit{i}-1\right]$ must contain the value of $\frac{\partial {f}_{\mathit{i}}}{\partial {x}_{\mathit{j}}}$ at the point $x$, for $\mathit{i}=1,2,\dots ,n$ and $\mathit{j}=1,2,\dots ,n$.
On final exit: the orthogonal matrix $Q$ produced by the $QR$ factorization of the final approximate Jacobian, stored by columns.
6: $\mathbf{xtol}$double Input
On initial entry: the accuracy in x to which the solution is required.
Suggested value: $\sqrt{\epsilon }$, where $\epsilon$ is the machine precision returned by X02AJC.
Constraint: ${\mathbf{xtol}}\ge 0.0$.
7: $\mathbf{scale_mode}$Nag_ScaleType Input
On initial entry: indicates whether or not you have provided scaling factors in diag.
If ${\mathbf{scale_mode}}=\mathrm{Nag_ScaleProvided}$, the scaling must have been supplied in diag.
Otherwise, if ${\mathbf{scale_mode}}=\mathrm{Nag_NoScaleProvided}$, the variables will be scaled internally.
Constraint: ${\mathbf{scale_mode}}=\mathrm{Nag_NoScaleProvided}$ or $\mathrm{Nag_ScaleProvided}$.
8: $\mathbf{diag}\left[{\mathbf{n}}\right]$double Input/Output
On initial entry: if ${\mathbf{scale_mode}}=\mathrm{Nag_ScaleProvided}$, diag must contain multiplicative scale factors for the variables.
If ${\mathbf{scale_mode}}=\mathrm{Nag_NoScaleProvided}$, diag need not be set.
Constraint: if ${\mathbf{scale_mode}}=\mathrm{Nag_ScaleProvided}$,${\mathbf{diag}}\left[\mathit{i}-1\right]>0.0$, for $\mathit{i}=1,2,\dots ,n$.
On intermediate exit: diag must not be changed.
On final exit: the scale factors actually used (computed internally if ${\mathbf{scale_mode}}=\mathrm{Nag_NoScaleProvided}$).
9: $\mathbf{factor}$double Input
On initial entry: a quantity to be used in determining the initial step bound. In most cases, factor should lie between $0.1$ and $100.0$. (The step bound is ${\mathbf{factor}}×{‖{\mathbf{diag}}×{\mathbf{x}}‖}_{2}$ if this is nonzero; otherwise the bound is factor.)
Suggested value: ${\mathbf{factor}}=100.0$.
Constraint: ${\mathbf{factor}}>0.0$.
10: $\mathbf{r}\left[{\mathbf{n}}×\left({\mathbf{n}}+1\right)/2\right]$double Input/Output
On initial entry: need not be set.
On intermediate exit: must not be changed.
On final exit: the upper triangular matrix $R$ produced by the $QR$ factorization of the final approximate Jacobian, stored row-wise.
11: $\mathbf{qtf}\left[{\mathbf{n}}\right]$double Input/Output
On initial entry: need not be set.
On intermediate exit: must not be changed.
On final exit: the vector ${Q}^{\mathrm{T}}f$.
12: $\mathbf{iwsav}\left[17\right]$Integer Communication Array
13: $\mathbf{rwsav}\left[4×{\mathbf{n}}+10\right]$double Communication Array
The arrays iwsav and rwsav MUST NOT be altered between calls to c05rdc.
14: $\mathbf{fail}$NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

## 6Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
On entry, argument $⟨\mathit{\text{value}}⟩$ had an illegal value.
NE_DIAG_ELEMENTS
On entry, ${\mathbf{scale_mode}}=\mathrm{Nag_ScaleProvided}$ and diag contained a non-positive element.
NE_INT
On entry, ${\mathbf{irevcm}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{irevcm}}=0$, $1$, $2$ or $3$.
On entry, ${\mathbf{n}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{n}}>0$.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
NE_NO_IMPROVEMENT
The iteration is not making good progress, as measured by the improvement from the last $⟨\mathit{\text{value}}⟩$ iterations. This failure exit may indicate that the system does not have a zero, or that the solution is very close to the origin (see Section 7). Otherwise, rerunning c05rdc from a different starting point may avoid the region of difficulty.
The iteration is not making good progress, as measured by the improvement from the last $⟨\mathit{\text{value}}⟩$ Jacobian evaluations. This failure exit may indicate that the system does not have a zero, or that the solution is very close to the origin (see Section 7). Otherwise, rerunning c05rdc from a different starting point may avoid the region of difficulty.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
NE_REAL
On entry, ${\mathbf{factor}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{factor}}>0.0$.
On entry, ${\mathbf{xtol}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{xtol}}\ge 0.0$.
NE_TOO_SMALL
No further improvement in the solution is possible. xtol is too small: ${\mathbf{xtol}}=⟨\mathit{\text{value}}⟩$.

## 7Accuracy

If $\stackrel{^}{x}$ is the true solution and $D$ denotes the diagonal matrix whose entries are defined by the array diag, then c05rdc tries to ensure that
 $‖D(x-x^)‖2 ≤ xtol × ‖Dx^‖2 .$
If this condition is satisfied with ${\mathbf{xtol}}={10}^{-k}$, then the larger components of $Dx$ have $k$ significant decimal digits. There is a danger that the smaller components of $Dx$ may have large relative errors, but the fast rate of convergence of c05rdc usually obviates this possibility.
If xtol is less than machine precision and the above test is satisfied with the machine precision in place of xtol, then the function exits with ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NE_TOO_SMALL.
Note:  this convergence test is based purely on relative error, and may not indicate convergence if the solution is very close to the origin.
The convergence test assumes that the functions and the Jacobian are coded consistently and that the functions are reasonably well behaved. If these conditions are not satisfied, then c05rdc may incorrectly indicate convergence. The coding of the Jacobian can be checked using c05zdc. If the Jacobian is coded correctly, then the validity of the answer can be checked by rerunning c05rdc with a lower value for xtol.

## 8Parallelism and Performance

c05rdc is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
c05rdc 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 function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

The time required by c05rdc to solve a given problem depends on $n$, the behaviour of the functions, the accuracy requested and the starting point. The number of arithmetic operations executed by c05rdc is approximately $11.5×{n}^{2}$ to process each evaluation of the functions and approximately $1.3×{n}^{3}$ to process each evaluation of the Jacobian. The timing of c05rdc is strongly influenced by the time spent evaluating the functions.
Ideally the problem should be scaled so that, at the solution, the function values are of comparable magnitude.

## 10Example

This example determines the values ${x}_{1},\dots ,{x}_{9}$ which satisfy the tridiagonal equations:
 $(3-2x1)x1-2x2 = -1, -xi-1+(3-2xi)xi-2xi+1 = -1, i=2,3,…,8 -x8+(3-2x9)x9 = -1.$

### 10.1Program Text

Program Text (c05rdce.c)

None.

### 10.3Program Results

Program Results (c05rdce.r)