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Derivative-free Methods
using
Linesearch Techniques
Stefano Lucidi
P. Tseng
L. Grippo
joint works with
(the father linesearch approach)
M. Sciandrone
G. Liuzzi
F. Lampariello
V. Piccialli
(in order of appearance in this research activity)
F. Rinaldi G. Fasano
PROBLEM DEFINITION:
0)( s.t.
)( min
xg
xf
1n , : CfRRf 1
imn , : CgRRg
n Rx
i, gf are not available
MOTIVATIONS:
In many engineering problems the objective and constraint function values are obtained by
direct measurements
complex simulation programs
first order derivatives can be often neither explicitly calculated nor approximated
MOTIVATIONS:
In fact
the mathematical representations of the objective function and the constraints are not available
the source codes of the programs are not available
the values of the objective function and the constraints can be affected by the presence of noise
the evaluations of the objective function and the constraints can be very expensive
MOTIVATIONS:
the mathematical representations of the objective function and the constraints are not available
the first order derivatives the objective function and the constraints can not be computed analytically
MOTIVATIONS:
the source codes of the programs are not available
the automatic differentiation techniques can not be applied
MOTIVATIONS:
the evaluations of the objective function and the constraints can be very expensive
the finite difference approximations can be too expensive(they need n function evaluations at least)
MOTIVATIONS:
finite difference approximations can produce very wrong estimates of the first order derivatives
the values of the objective function and the constraints can be affected by the presence of noise
NUMERICAL EXPERIENCE:
we considered 41 box constrained standard test problems
we perturbed such problems in the following way:
)N (0, ,1)( )(~
2 xfxf
where denotes a Gaussian distributed random number
with zero mean and variance
)N (0, 2 2
NUMERICAL EXPERIENCE:
we considered two codes:
Number of Failures
0 2
DF_box = derivative-free method
E04UCF = NAG subroutine using finite-differences gradients
-92 01
DF_box
E04UCF
3 3
2 23
GLOBALLY CONVERGENT DF METHODS
Direct search methods use only function values
- pattern search methods where the function is evaluated
on specified geometric patterns
- line search methods which use one-dimensional minimization
along suitable search directions
Modelling methods approximate the functions by suitable
models which are progressively built and updated
UNCONSTRAINED MINIMIZATION PROBLEMS
n s.t.
)( min
Rx
xf
f
1n , : CfRRf
n Rx
is not available
: 0n
0 xfxfRxL is compact
THE ROLE OF THE GRADIENT
kkk1k dxx
characterizes accurately the local behaviour of f allows us
to determine an "efficient" descent direction kd
f
to determine a "good" step length along the directionk
THE ROLE OF THE GRADIENT
iT
i
efx
f
is the directional derivatives of alongf ie
f provides the rates of change of along the 2n directions f ie
f characterizes accurately the local behaviour of f
0
f
0
0 0
0
nT
1T
nT
1T
ef
efef
ef
HOW TO OVERCOME THE LACK OF GRADIENT
r,1 ,i ,ik p the local behaviour of along f
should be indicative of the whole local behaviour of f
0
f
0
0
rk
T
1k
T
pf
pf
a set of directions can be associated at each
kxr,1 ,i ,ik p
ASSUMPTION D
r,1 ,i , ik pGiven , the bounded sequences are
such that
k x
0,0 minlimr
1i
ik
Tk
k
pxf
0 lim kk
xf
EXAMPLES OF SETS OF DIRECTIONS
n,1,i ,
n,1,i ,ik
nik
ik
pp
p are linearly independent and bounded
22k ep
11k ep
,24k ep
13 ep
EXAMPLES OF SETS OF DIRECTIONS (Lewis,Torczon)
nrk
1k
ik
,,
r,1,i ,
Rppcone
p
are bounded
22k ep
11k ep
1
13p
EXAMPLES OF SETS OF DIRECTIONS
kx
1kv
2kv
,2k
1kk vfvfxf
11k ep
22k ep
22k ep
UNCONSTRAINED MINIMIZATION PROBLEMS
Assumption D ensures that, performing finer and finer sampling
of along it is possible:
f r,1 ,i ,ik p
- either to realize that the point is a good approximation of a
stationary point of fkx
- or to find a point where is decreased f1kx
GLOBAL CONVERGENCE
By Assumption D we have:
0 f
0
0
rk
T
1k
T
pf
pf
0 , 0
0 , 0
rkk
rk
rkk
1kk
1k
1kk
xfpxf
xfpxf
0
0
rk
T
1k
T
pf
pf
rk
krk
rkkr
kT
1k
k1k
1kk1
kT
xfpxfpf
xfpxfpf
GLOBAL CONVERGENCE
By using satisfying Assumption D it is possible: r,1 ,i , ik p
to characterize the global convergence of a sequence of points kx
by means
the existence of suitable sequences of failures in decreasing the
objective function along the directions r,1 ,i , ik p f
GLOBAL CONVERGENCE
By Assumption D we have:
0 f
0
0
rk
T
1k
T
pf
pf
r 1,i 0,
0 , 0
0 , 0
kik
rk
rk
rk
rk
rk
1k
1k
1k
1k
1k
xy
yfpyf
yfpyf
0
0
rk
T
1k
T
pf
pf
rk
krk
rkkr
kT
1k
k1k
1kk1
kT
xfpxfpf
xfpxfpf
PROPOSITION Let and be such that: r,1,i , i
k p kx
r,1,i , ik p
k1k xfxf -
- satisfy Assumption D
then
0 lim kk
xf
- there exist sequences of points and scalars
such that
r,1,i , ik
0 lim
0 lim
kik
k
ik
ik
ik
ik
ik
ik
k
xy
oyfpyf
y ik
GLOBAL CONVERGENCE
• the sampling of along all the directions can be
distributed along the iterations of the algorithm
r,1,i , ik p f
• the Proposition characterizes in “some sense” the requirements on the accettable samplings of along the directions that guarantee the global convergence
r,1,i , ik p f
• it is not necessary to perform at each point a sampling
of along all the directions f r,1,i , ik p
kx
GLOBAL CONVERGENCE
The use of directions satisfying Condition D and the result of producing
sequences of points satisfying the hypothesis of the Proposition are
the common elements of
all the globally convergent direct search methods
The direct search methods can divided in
- pattern search methods
- line search methods
PATTERN SEARCH METHODS
Cons: all the points produced must lie in a suitable lattice this implies - additional assumptions on the search directions
- restrictions on the choiches of the steplenghts
Pros: they require that the new point produces a simple decrease
of f
(in the line search methods the new point must guarantees
a “sufficient” decrease of ) f
(in the line search methods no additional requiriments
respect to Assumption D and the assumptions of the Proposition)
LINESEARCH TECHNIQUES
(0,1) ,22 k
T
kkkkkk
k
T
kkkkkk
dxfxfdxf
dxfxfdxf
kk dxf
k
T
kk dxfxf
k k2
(0,1) ,22
2
kkkkkk
2
kkkkkk
dxfdxf
dxfdxf
kk dxf
2k dxf
LINESEARCH TECHNIQUES
k k2
ALGORITHM DF
STEP 1 Compute satisfying Assumption D
rk
1k , , pp
Minimization of along STEP 2 r
k1k , , pp f
STEP 3 Compute and set k=k+1
1rk1k
yx
) ( k1k
ik
ik
ik
1ik xypyy
STEP 2
The aim of this step is:
- to detect the “promising” directions, the direction along which the function decreases “sufficiently”
- to compute steplenghts along these directions which guarantee both a “sufficiently” decrease of the function and a “sufficient” moving from the previous point
ik
ik pyf
2ik
ik pyf
LINESEARCH TECHNIQUE
ik
~ik
~
)~,( ik
ik p
2~ ~
0
ik
i1k
ik
1ik
ik
yy
ik
ik pyf
2ik
ik pyf
LINESEARCH TECHNIQUE
ik
~8ik
~2 ik
~4ik
~
)~,( ik
ik p
ik
i1k
ik
ik
ik
1ik
ik
ik
~
~4
pyy
STEP 2
ik
~ The value of the initial step along the i-th direction derives from
the linesearch performed along the i-th direction at the previuos
iteration
If the set of search directions does not depend on the iteration
namely
the scalar should be representative of the behaviour of the objective
function along the i-th direction
r,1 ,i ,iik pp
ik
~ip
STEP 3
set k=k+1 and go to Step 1
Find such that 1kx 1rk1k
yfxf
otherwise set
1rk1k
yx
At Step 3, every approximation technique can be used to produce
a new better point
GLOBAL CONVERGENCE
THEOREM Let be the sequence of points produced by DF
Algorithm then there exists an accomulation point of and every
accumulation points of is a stationary point of the objective
function
k x k x
k xf
LINEARLY CONSTRAINED MINIMIZATION PROBLEMS
1n , : CfRRf
mnmn , , RbRARx
f is not available
: 00 xfxfFxL is compactis compact
bAxRxF : n
bAx
xf
s.t.
)( min(LCP)
LINEARLY CONSTRAINED MINIMIZATION PROBLEMS
Fx Given a feasible point it is possible to define
• the set of the indeces of the active constraints
jTj :m,1 ,j I bxax
• the set of the feasible directions
I(x )j ,0 : Tj
n daRdxT
LINEARLY CONSTRAINED MINIMIZATION PROBLEMS
is a stationary point
for Problem (LCP)
Fx * *T* T ,0 xppxf
is a stationary point
for Problem (LCP)
Fx *
*r1 T,,c o n e xpp
0
0
rT
1T
pf
pf
LINEARLY CONSTRAINED MINIMIZATION PROBLEMS
is a stationary point
for Problem (LCP)
Fx k
krk
1k T,,c o n e xpp
0
0
rk
Tk
1k
Tk
pxf
pxf
0 , 0
0 , 0
rkk
rk
rkk
1kk
1k
1kk
xfpxf
xfpxf
0
0
rk
Tk
1k
Tk
pxf
pxf
xxxx
TT lim kk
LINEARLY CONSTRAINED MINIMIZATION PROBLEMS
• an estimate of the set of the indeces of the active constraints
jTj :m,1 ,j ,I bxax
• an estimate of the set of the feasible directions
),I(j ,0 : , Tj
n xdaRdxT
Fx Given and it is possible to define 0
• has good properties which allow us to define globally convergent algorithms
kk ,T x
ASSUMPTION D2 (an example)
, , krk
1k pp Given and the set of directions
with satisfies:
k x 0
kik r,1,i ,1 p
kr is uniformly bounded
0 , ,T,T, , kkrk
1k
k xxppcone
,T k x
1kp
2kp
ALGORITHM DFL
STEP 1 Compute satisfying Assumption D2 kr
k1k , , pp
Minimization of along
STEP 2 kr
k1k , , pp f
STEP 3 Compute the new point and set k=k+11kx
GLOBAL CONVERGENCE
THEOREM Let be the sequence of points produced by DFL
Algorithm then there exists an accomulation point of and every
accumulation points of is a stationary point for Problem
(LCP)
k x k x
k x
BOX CONSTRAINED MINIMIZATION PROBLEMS
1n , : CfRRf
nnn , , RuRlRx
f is not available
: 00 xfxfFxL is compact
uxlRxF : n
uxl
xf
s.t.
)( min(BCP)
nn11 ,,,, eeee satisfies Assumption D2 the set
NONLINEARLY CONSTRAINED MINIMIZATION PROBLEMS
bAx
xg
xf
0 s.t.
)( min(NCP)
mnmn , , RbRARx
p,1 ,i , , : 1i
hn CgRRg
f is not available
p,1 ,i , i g are not available
1n , : CfRRf
mnmn , , RbRARx
p,1 ,i , , : 1i
hn CgRRg
NONLINEARLY CONSTRAINED MINIMIZATION PROBLEMS
Fx~
and given a point
jTj :m,1 ,j I bxax
I(x )j ,0 : Tj
n daRdxT
bAxxgRxF ,0)(: n
We define
bAxRxF : ~
n
NONLINEARLY CONSTRAINED MINIMIZATION PROBLEMS
ASSUMPTION A1 F
~ The set
is compact
Fx~
For every 0 :i , 0 i
Ti xgdxg
xTd ASSUMPTION A2 there exists a vector such that
Assumption A1
boundeness of the iterates
Assumption A2
existence and boundeness of the Lagrangemultipliers
NONLINEARLY CONSTRAINED MINIMIZATION PROBLEMS
0, max1
)();P(h
1i
qi
xgxfx
We consider the following continuously differentiable
penalty function:
where
0
1q
(penalty parameter)
NONLINEARLY CONSTRAINED MINIMIZATION PROBLEMS
bAx
xg
xf
0 s.t.
)( min
bAx
x
s.t.
;P min
? 0k ?
ALGORITHM DFN
STEP 1 Compute satisfying Assumption D2 kr
k1k , , pp
Minimization of along
STEP 2 kr
k1k , , pp k;P x
STEP 3 Compute the new point and set k=k+11k1k , x
new STEP 3 ( )
set k=k+1 and go to Step 1
Find such that
Fx~
1k k1r
kk1k ;P;P yx
otherwise set
1rk1k
yx
0 ,1,0 ,0 k s
if
sk
ik
r,1,i ~ max
k
set
k1k
otherwise set k1k
and
k
h
1iki0, max
xg then
new STEP 3
is reduced whenever a better approximation of a stationary point of the penalty function has
been obtained
k
can be viewed as stationarity measure
ikr,1,i
~ maxk
0~ max ik
r,1,i k
GLOBAL CONVERGENCE
THEOREM Let be the sequence of points produced by DFN
Algorithm then there exists an accomulation point of which is
a stationary point for Problem (LCP)
k x k x
MIXED NONLINEARLY MINIMIZATION PROBLEMS
uxl
xg
xf
0 s.t.
)( min
(MNCP)
zi i , IZx
zinn i , : ,0)(: IZxRxuxlxgRxF
We define
uxlRxF : ~
n
zz In n. discr. var. cc Inn n. cont. var.
MIXED NONLINEARLY MINIMIZATION PROBLEMS
We define
c
zc
iiciiziic , ,
I
II x
xhxhxxxx
1 , : ,
0 0,
~ ,0
zzccn
T
c
T
cc
xxxxRxFxxfxf
xg
Fxxxxgxf
Fx is a stationary point of Problem MNLP if there exists m R such that:
ALGORITHM MDFN
STEP 1 Compute
, , nnk
11k epep
Mixed Minimization of along STEP 2 n
k1k , , pp k;P x
STEP 3 Compute the new point and set k=k+11k1k , x
ALGORITHM MDFN
STEP 1 Compute
, , nnk
11k epep
STEP 2
STEP 3 Compute the new point and set k=k+11k1k , x
i
kpa continuous linesearch along
cIiIf perform
i
kpa discrete linesearch along
zIiIf perform
Continuous linesearch
Fy~1i
k
Continuous linesearch of MDFN = linesearch of DFN
it produces the point
i
k
i
k
i
k
1i
k pyy
ik
ik pyf
||ik
ik yf
LINESEARCH TECHNIQUE
ik
~ik
~
)~,( ik
ik p
)2~ max(1,~
0
ik
i1k
ik
1ik
ik
yy 2 ik
i1k
ik
ik pyf
||i1k
ik yf
LINESEARCH TECHNIQUE
ik
~ik
~
)~,( ik
ik p
)2~ max(1,~
0
ik
i1k
ik
1ik
ik
yy 2 ik
i1k
ik
ik pyf
LINESEARCH TECHNIQUE
ik
~8ik
~2 ik
~4ik
~
)~,( ik
ik p
ik
i1k
ik
ik
ik
1ik
ik
ik
~
~4
pyy
||ik
ik yf
ik
i1k
MIXED NONLINEARLY MINIMIZATION PROBLEMS
or every accumulation point of the sequence produced by
the algorithm, satisfies:
ASSUMPTION A3
Either the nonlinear constraints functions do not
depend on the integer variables
m ,1 ,...,i, i g
zi i , Ix
Fx
1 , : ~zzcc
n xxxxRxFx
are such that
p1,...,i 0 , ~ i
xg
GLOBAL CONVERGENCE
THEOREM Let be the sequence of points produced by MDFN
Algorithm then there exists an accomulation point of which is a
stationary point for Problem (MNCP)
k x k x
MIXED NONLINEARLY MINIMIZATION PROBLEMS
More complex (and expensive) derivative-free algorithms allows us
to determine “better” stationary points
to tackle “more difficult” mixed nonlinear optimization problems
MIXED NONLINEARLY MINIMIZATION PROBLEMS
to determine “better” stationary points for Problem (MNCP)
1 , :~ ,~
0 0,
~ ,0
zzccn
T
c
T
cc
xxxxRxFxxfxf
xg
Fxxxxgxf
~
~ : 1 , :~zzcc
n
x
xfxfxxxxRxFx
satisfies the KKT conditions w.r.t. cx
z
y
x
discrete general variables
continuous variables
discrete dimensional variables
Discrete dimensional variables z: Vector of discrete variables which determine the number of continuous and discrete variables
Three different sets of variables:
to tackle “more difficult” mixed nonlinear optimization problems
znx
zny
nz
d
c
z
Rzyx
zy
z
zyxf
),(
)(
),,(min
HARD MIXED NONLINEARLY MINIMIZATION PROBLEMS
The feasible set of y depends on the dimensional variables z
The feasible set of x depends on the discrete variables y and on the dimensional variables z
(Hard-MNCP)
}2)-(yx,y5)-max{(x min 2222
NONSMOOTH MINIMIZATION PROBLEMS
}2)-(yx,y5)-max{(x min 2222
NONSMOOTH MINIMIZATION PROBLEMS
the cone of descent directions can be made arbitrarily narrow
NONSMOOTH MINIMIZATION PROBLEMS
Possible approaches:
smoothing techniques
“larger” set of search directions
NONSMOOTH MINIMIZATION PROBLEMS
smoothing techniques
2n , : CfRRf
mnmn , , RbRARx
bAx
xf
s.t.
)(max min iqi1
NONSMOOTH MINIMIZATION PROBLEMS
)(max )( iqi1 xfxf
xx ln x x,
x ln x,
i q
1 i
i q
1 i
ffxpeff
fxpef
0
ALGORITHM DFN
STEP 1 Compute satisfying Assumption D2 kr
k1k , , pp
Minimization of along
STEP 2 kr
k1k , , pp k;xf
STEP 3 Compute the new point and set k=k+11k1k , x
new STEP 3
set k=k+1 and go to Step 1
Find such that
Fx~
1k k1r
kk1k ;; yfxf
otherwise set
1rk1k
yx
set
21 i 21 i i k 1k )(,)~( max , min
new STEP 3
is reduced whenever a better approximation of a stationary point of the penalty function has
been obtained
k
can be viewed as stationarity measure
ikr,1,i
~ maxk
0~ max ik
r,1,i k
GLOBAL CONVERGENCE
THEOREM Let be the sequence of points produced by the
Algorithm then there exists an accomulation point of which is
a stationary point for the MinMax Problem
k x k x
NONSMOOTH MINIMIZATION PROBLEMS
“larger” set of search directions
uxl
xg
xf
0 s.t.
)( min(NCP)
nnn , , RuRlRx
RRf : n
p,1,i , : hni RRg
locally Lipschitz-continuous
locally Lipschitz-continuous
0, max1
)();Z(h
1ii
xgxfx
We consider the following nonsmooth penalty function:
where
0 (penalty parameter)
NONSMOOTH MINIMIZATION PROBLEMS
NONSMOOTH MINIMIZATION PROBLEMS
ASSUMPTION A1 F
~ The set
is compact
Fx~
For every 0 :i , 0 i
Ti xgdxg
xTd ASSUMPTION A2 there exists a vector such that
bAx
xg
xf
0 s.t.
)( min
bAx
x
s.t.
; Zmin
],0(
NONSMOOTH MINIMIZATION PROBLEMS
, , krk
1k pp set of search directions which are
asintotically dense in the unit sphere
It is possible to define algorithms globally convergent towards
NONSMOOTH MINIMIZATION PROBLEMS
stationary points (in the Clarke sense)
by assuming that the algorithms use
NONSMOOTH MINIMIZATION PROBLEMS
Multiobjective optimization problem
(working in progress)
uxl
xg
xfxf
0 s.t.
)( ,),( min q1
nnn , , RuRlRx
q,1,j , : nj RRf
p,1,i , : ni RRg
locally Lipschitz-continuous
locally Lipschitz-continuous
NONSMOOTH MINIMIZATION PROBLEMS
Bilevel optimization problem
(working in progress)
yy
xx
uyl
yxg
yxfy
uxl
yxG
yxF
0, s.t.
),( min arg
where
0, s.t.
),( min
Our DF-codes are available at:
http://www.dis.uniroma1.it/~lucidi/DFL
Thank your
for your attention
n. m
agne
ts=
3
n. rings=6
n.m
agne
ts=4
half magnet
Optimal Design of Magnetic Resonance apparatus
magnetsn
ringsnz
.
.
Design Variables
Positions of the rings along the X-axis
X
x1 x2 x3 x4 x5 x6
Angular positions of each row of small magnets 1 2
3
4
Design Variables
Offsets of the 4 outermost rings w.r.t. the 2 innermost ones
Xb1 b2 b3 b4
r
Radius of magnets (integer values)
ry
2
1
2
1
2
1
b
b
x
x
x
Objective Function
The objective function measures the non-uniformity of the
magnetic field within a specified target region which is
21
2)(2)(2)(
xB
xBxBxBxBxU
Z
N
i
iY
iXZ
iZ
p
p
N
i
iZ
Z N
xBxB
p
1
)(
Magnetic field as uniform as possible and directed along the Z axis
nr=5nm=3r=22
f=51 ppm
Starting point (commercial devices)
nr=7nm=3r=27
f=18 ppm
Final point
Magnetic Resonance Results
*
**
xB
xBxB
Z
ZZ Behavior of on the ZY plane
51ppm configuration 18ppm configuration