NAG Library Routine Document
E04JYF
1 Purpose
E04JYF is an easy-to-use quasi-Newton algorithm for finding a minimum of a function Fx1,x2,…,xn, subject to fixed upper and lower bounds of the independent variables x1,x2,…,xn, using function values only.
It is intended for functions which are continuous and which have continuous first and second derivatives (although it will usually work even if the derivatives have occasional discontinuities).
2 Specification
SUBROUTINE E04JYF ( |
N, IBOUND, FUNCT1, BL, BU, X, F, IW, LIW, W, LW, IUSER, RUSER, IFAIL) |
INTEGER |
N, IBOUND, IW(LIW), LIW, LW, IUSER(*), IFAIL |
REAL (KIND=nag_wp) |
BL(N), BU(N), X(N), F, W(LW), RUSER(*) |
EXTERNAL |
FUNCT1 |
|
3 Description
E04JYF is applicable to problems of the form:
when derivatives of
Fx are unavailable.
Special provision is made for problems which actually have no bounds on the xj, problems which have only non-negativity bounds and problems in which l1=l2=⋯=ln and u1=u2=⋯=un. You must supply a subroutine to calculate the value of Fx at any point x.
From a starting point you supplied there is generated, on the basis of estimates of the gradient and the curvature of Fx, a sequence of feasible points which is intended to converge to a local minimum of the constrained function. An attempt is made to verify that the final point is a minimum.
A typical iteration starts at the current point
x where
nz (say) variables are free from both their bounds. The projected gradient vector
gz, whose elements are finite difference approximations to the derivatives of
Fx with respect to the free variables, is known. A unit lower triangular matrix
L and a diagonal matrix
D (both of dimension
nz), such that
LDLT is a positive definite approximation of the matrix of second derivatives with respect to the free variables (i.e., the projected Hessian) are also held. The equations
are solved to give a search direction
pz, which is expanded to an
n-vector
p by an insertion of appropriate zero elements. Then
α is found such that
Fx+αp is approximately a minimum (subject to the fixed bounds) with respect to
α;
x is replaced by
x+αp, and the matrices
L and
D are updated so as to be consistent with the change produced in the estimated gradient by the step
αp. If any variable actually reaches a bound during the search along
p, it is fixed and
nz is reduced for the next iteration. Most iterations calculate
gz using forward differences, but central differences are used when they seem necessary.
There are two sets of convergence criteria – a weaker and a stronger. Whenever the weaker criteria are satisfied, the Lagrange multipliers are estimated for all the active constraints. If any Lagrange multiplier estimate is significantly negative, then one of the variables associated with a negative Lagrange multiplier estimate is released from its bound and the next search direction is computed in the extended subspace (i.e., nz is increased). Otherwise minimization continues in the current subspace provided that this is practicable. When it is not, or when the stronger convergence criteria are already satisfied, then, if one or more Lagrange multiplier estimates are close to zero, a slight perturbation is made in the values of the corresponding variables in turn until a lower function value is obtained. The normal algorithm is then resumed from the perturbed point.
If a saddle point is suspected, a local search is carried out with a view to moving away from the saddle point. A local search is also performed when a point is found which is thought to be a constrained minimum.
4 References
Gill P E and Murray W (1976) Minimization subject to bounds on the variables
NPL Report NAC 72 National Physical Laboratory
5 Parameters
- 1: N – INTEGERInput
On entry: the number n of independent variables.
Constraint:
N≥1.
- 2: IBOUND – INTEGERInput
On entry: indicates whether the facility for dealing with bounds of special forms is to be used.
It must be set to one of the following values:
- IBOUND=0
- If you are supplying all the lj and uj individually.
- IBOUND=1
- If there are no bounds on any xj.
- IBOUND=2
- If all the bounds are of the form 0≤xj.
- IBOUND=3
- If l1=l2=…=ln and u1=u2=…=un.
- 3: FUNCT1 – SUBROUTINE, supplied by the user.External Procedure
You must supply
FUNCT1 to calculate the value of the function
Fx at any point
x. It should be tested separately before being used with E04JYF (see the
E04 Chapter Introduction).
The specification of
FUNCT1 is:
INTEGER |
N, IUSER(*) |
REAL (KIND=nag_wp) |
XC(N), FC, RUSER(*) |
|
- 1: N – INTEGERInput
On entry: the number n of variables.
- 2: XC(N) – REAL (KIND=nag_wp) arrayInput
On entry: the point x at which the function value is required.
- 3: FC – REAL (KIND=nag_wp)Output
On exit: the value of the function F at the current point x.
- 4: IUSER(*) – INTEGER arrayUser Workspace
- 5: RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
-
FUNCT1 is called with the parameters
IUSER and
RUSER as supplied to E04JYF. You are free to use the arrays
IUSER and
RUSER to supply information to
FUNCT1 as an alternative to using COMMON global variables.
FUNCT1 must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which E04JYF is called. Parameters denoted as
Input must
not be changed by this procedure.
- 4: BL(N) – REAL (KIND=nag_wp) arrayInput/Output
On entry: the lower bounds
lj.
If
IBOUND is set to
0, you must set
BLj to
lj, for
j=1,2,…,n. (If a lower bound is not specified for a particular
xj, the corresponding
BLj should be set to
-106.)
If
IBOUND is set to
3, you must set
BL1 to
l1; E04JYF will then set the remaining elements of
BL equal to
BL1.
On exit: the lower bounds actually used by E04JYF.
- 5: BU(N) – REAL (KIND=nag_wp) arrayInput/Output
On entry: the upper bounds
uj.
If
IBOUND is set to
0, you must set
BUj to
uj, for
j=1,2,…,n. (If an upper bound is not specified for a particular
xj, the corresponding
BUj should be set to
106.)
If
IBOUND is set to
3, you must set
BU1 to
u1; E04JYF will then set the remaining elements of
BU equal to
BU1.
On exit: the upper bounds actually used by E04JYF.
- 6: X(N) – REAL (KIND=nag_wp) arrayInput/Output
On entry: Xj must be set to an estimate of the jth component of the position of the minimum, for j=1,2,…,n.
On exit: the lowest point found during the calculations. Thus, if IFAIL=0 on exit, Xj is the jth component of the position of the minimum.
- 7: F – REAL (KIND=nag_wp)Output
On exit: the value of
Fx corresponding to the final point stored in
X.
- 8: IW(LIW) – INTEGER arrayOutput
On exit: if
IFAIL=0,
3 or
5, the first
N elements of
IW contain information about which variables are currently on their bounds and which are free. Specifically, if
xi is:
– |
fixed on its upper bound, IWi is -1; |
– |
fixed on its lower bound, IWi is -2; |
– |
effectively a constant (i.e., lj=uj), IWi is -3; |
– |
free, IWi gives its position in the sequence of free variables. |
In addition, IWN+1 contains the number of free variables (i.e., nz). The rest of the array is used as workspace.
- 9: LIW – INTEGERInput
On entry: the dimension of the array
IW as declared in the (sub)program from which E04JYF is called.
Constraint:
LIW≥N+2.
- 10: W(LW) – REAL (KIND=nag_wp) arrayOutput
On exit: if IFAIL=0, 3 or 5, Wi contains a finite difference approximation to the
ith element of the projected gradient vector gz, for i=1,2,…,N. In addition, WN+1 contains an estimate of the condition number of the projected Hessian matrix (i.e., k). The rest of the array is used as workspace.
- 11: LW – INTEGERInput
On entry: the dimension of the array
W as declared in the (sub)program from which E04JYF is called.
Constraint:
LW≥maxN×N-1/2+12×N,13.
- 12: IUSER(*) – INTEGER arrayUser Workspace
- 13: RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
-
IUSER and
RUSER are not used by E04JYF, but are passed directly to
FUNCT1 and may be used to pass information to this routine as an alternative to using COMMON global variables.
- 14: IFAIL – INTEGERInput/Output
-
On entry:
IFAIL must be set to
0,
-1 or 1. If you are unfamiliar with this parameter you should refer to
Section 3.3 in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value
-1 or 1 is recommended. If the output of error messages is undesirable, then the value
1 is recommended. Otherwise, because for this routine the values of the output parameters may be useful even if
IFAIL≠0 on exit, the recommended value is
-1.
When the value -1 or 1 is used it is essential to test the value of IFAIL on exit.
On exit:
IFAIL=0 unless the routine detects an error or a warning has been flagged (see
Section 6).
6 Error Indicators and Warnings
If on entry
IFAIL=0 or
-1, explanatory error messages are output on the current error message unit (as defined by
X04AAF).
Note: E04JYF may return useful information for one or more of the following detected errors or warnings.
Errors or warnings detected by the routine:
- IFAIL=1
On entry, | N<1, |
or | IBOUND<0, |
or | IBOUND>3, |
or | IBOUND=0 and BLj>BUj for some j, |
or | IBOUND=3 and BL1>BU1, |
or | LIW<N+2, |
or | LW < max13,12×N+N×N-1/2 . |
- IFAIL=2
There have been
400×n function evaluations, yet the algorithm does not seem to be converging. The calculations can be restarted from the final point held in
X. The error may also indicate that
Fx has no minimum.
- IFAIL=3
The conditions for a minimum have not all been met but a lower point could not be found and the algorithm has failed.
- IFAIL=4
An overflow has occurred during the computation. This is an unlikely failure, but if it occurs you should restart at the latest point given in
X.
- IFAIL=5
- IFAIL=6
- IFAIL=7
- IFAIL=8
There is some doubt about whether the point
x found by E04JYF is a minimum. The degree of confidence in the result decreases as
IFAIL increases. Thus, when
IFAIL=5 it is probable that the final
x gives a good estimate of the position of a minimum, but when
IFAIL=8 it is very unlikely that the routine has found a minimum.
- IFAIL=9
In the search for a minimum, the modulus of one of the variables has become very large
∼106. This indicates that there is a mistake in
FUNCT1, that your problem has no finite solution, or that the problem needs rescaling (see
Section 8).
- IFAIL=10
The computed set of forward-difference intervals (stored in W9×N+1,W9×N+2,…, W10×N) is such that Xi+W9×N+i≤Xi for some i.
This is an unlikely failure, but if it occurs you should attempt to select another starting point.
If you are dissatisfied with the result (e.g., because
IFAIL=5,
6,
7 or
8), it is worth restarting the calculations from a different starting point (not the point at which the failure occurred) in order to avoid the region which caused the failure. If persistent trouble occurs and the gradient can be calculated, it may be advisable to change to a routine which uses gradients (see the
E04 Chapter Introduction).
7 Accuracy
A successful exit (
IFAIL=0) is made from E04JYF when (
B1,
B2 and
B3) or
B4 hold, and the local search confirms a minimum, where
- B1≡α
k
×p
k
<xtol+ε×1.0+x
k
- B2≡F
k
-F
k-1
<xtol2+ε×1.0+F
k
- B3≡gz
k
<ε1/3+xtol×1.0+F
k
- B4≡gz
k
<0.01×ε.
(Quantities with superscript
k are the values at the
kth iteration of the quantities mentioned in
Section 3,
xtol=100ε,
ε is the
machine precision and
. denotes the Euclidean norm. The vector
gz is returned in the array
W.)
If
IFAIL=0, then the vector in
X on exit,
xsol, is almost certainly an estimate of the position of the minimum,
xtrue, to the accuracy specified by
xtol.
If
IFAIL=3 or
5,
xsol may still be a good estimate of
xtrue, but the following checks should be made. Let
k denote an estimate of the condition number of the projected Hessian matrix at
xsol. (The value of
k is returned in
WN+1). If
(i) |
the sequence Fx
k
converges to Fxsol at a superlinear or a fast linear rate, |
(ii) |
gzxxol2<10.0×ε, and |
(iii) |
k<1.0/gzxsol, |
then it is almost certain that
xsol is a close approximation to the position of a minimum. When (ii) is true, then usually
Fxsol is a close approximation to
Fxtrue.
When a successful exit is made then, for a computer with a mantissa of t decimals, one would expect to get about t/2-1 decimals accuracy in x and about t-1 decimals accuracy in F, provided the problem is reasonably well scaled.
8 Further Comments
The number of iterations required depends on the number of variables, the behaviour of
Fx and the distance of the starting point from the solution. The number of operations performed in an iteration of E04JYF is roughly proportional to
n2. In addition, each iteration makes at least
m+1 calls of
FUNCT1, where
m is the number of variables not fixed on bounds. So, unless
Fx can be evaluated very quickly, the run time will be dominated by the time spent in
FUNCT1.
Ideally the problem should be scaled so that at the solution the value of Fx and the corresponding values of x1,x2,…,xn are each in the range -1,+1, and so that at points a unit distance away from the solution, F is approximately a unit value greater than at the minimum. It is unlikely that you will be able to follow these recommendations very closely, but it is worth trying (by guesswork), as sensible scaling will reduce the difficulty of the minimization problem, so that E04JYF will take less computer time.
9 Example
To minimize
subject to
starting from the initial guess
3,-1,0,1
.
9.1 Program Text
Program Text (e04jyfe.f90)
9.2 Program Data
None.
9.3 Program Results
Program Results (e04jyfe.r)