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Find minimum of unconstrained multivariable function
Finds the minimum of a problem specified by
where f(x) is a function that returns a scalar.
x is a vector or a matrix; see Matrix Arguments.
x = fminunc(fun,x0)
x = fminunc(fun,x0,options)
x = fminunc(problem)
[x,fval] = fminunc(...)
[x,fval,exitflag] = fminunc(...)
[x,fval,exitflag,output] = fminunc(...)
[x,fval,exitflag,output,grad] = fminunc(...)
[x,fval,exitflag,output,grad,hessian]
= fminunc(...)
fminunc attempts to find a minimum of a scalar function of several variables, starting at an initial estimate. This is generally referred to as unconstrained nonlinear optimization.
Note: Passing Extra Parameters explains how to pass extra parameters to the objective function, if necessary. 
x = fminunc(fun,x0) starts at the point x0 and attempts to find a local minimum x of the function described in fun. x0 can be a scalar, vector, or matrix.
x = fminunc(fun,x0,options) minimizes with the optimization options specified in options. Use optimoptions to set these options.
x = fminunc(problem) finds the minimum for problem, where problem is a structure described in Input Arguments.
Create the problem structure by exporting a problem from Optimization app, as described in Exporting Your Work.
[x,fval] = fminunc(...) returns in fval the value of the objective function fun at the solution x.
[x,fval,exitflag] = fminunc(...) returns a value exitflag that describes the exit condition.
[x,fval,exitflag,output] = fminunc(...) returns a structure output that contains information about the optimization.
[x,fval,exitflag,output,grad] = fminunc(...) returns in grad the value of the gradient of fun at the solution x.
[x,fval,exitflag,output,grad,hessian] = fminunc(...) returns in hessian the value of the Hessian of the objective function fun at the solution x. See Hessian.
Function Arguments contains general descriptions of arguments passed into fminunc. This section provides functionspecific details for fun, options, and problem:
The function to be minimized. fun is a function that accepts a vector x and returns a scalar f, the objective function evaluated at x. The function fun can be specified as a function handle for a file x = fminunc(@myfun,x0) where myfun is a MATLAB^{®} function such as function f = myfun(x) f = ... % Compute function value at x fun can also be a function handle for an anonymous function. x = fminunc(@(x)norm(x)^2,x0); If the gradient of fun can also be computed and the GradObj option is 'on', as set by options = optimoptions(@fminunc,'GradObj','on') then the function fun must return, in the second output argument, the gradient value g, a vector, at x. The gradient is the partial derivatives ∂f/∂x_{i} of f at the point x. That is, the ith component of g is the partial derivative of f with respect to the ith component of x. If the Hessian matrix can also be computed and the Hessian option is 'on', i.e., options = optimoptions(@fminunc,'GradObj','on','Hessian','on'), then the function fun must return the Hessian value H, a symmetric matrix, at x in a third output argument. The Hessian matrix is the second partial derivatives matrix of f at the point x. That is, the (i,j)th component of H is the second partial derivative of f with respect to x_{i} and x_{j}, ∂^{2}f/∂x_{i}∂x_{j}. The Hessian is by definition a symmetric matrix. Writing Scalar Objective Functions explains how to "conditionalize" the gradients and Hessians for use in solvers that do not accept them. Passing Extra Parameters explains how to parameterize fun, if necessary.  
options  Options provides the functionspecific details for the options values.  
problem  objective  Objective function  
x0  Initial point for x  
solver  'fminunc'  
options  Options created with optimoptions 
Function Arguments contains general descriptions of arguments returned by fminunc. This section provides functionspecific details for exitflag and output:
exitflag  Integer identifying the reason the algorithm terminated. The following lists the values of exitflag and the corresponding reasons the algorithm terminated.  
1  Magnitude of gradient smaller than the TolFun tolerance.  
2  Change in x was smaller than the TolX tolerance.  
3  Change in the objective function value was less than the TolFun tolerance.  
5  Predicted decrease in the objective function was less than the TolFun tolerance.  
0  Number of iterations exceeded MaxIter or number of function evaluations exceeded MaxFunEvals.  
1  Algorithm was terminated by the output function.  
3  Objective function at current iteration went below ObjectiveLimit.  
grad  Gradient at x  
hessian  Hessian at x  
output  Structure containing information about the optimization. The fields of the structure are  
iterations  Number of iterations taken  
funcCount  Number of function evaluations  
firstorderopt  Measure of firstorder optimality  
algorithm  Optimization algorithm used  
cgiterations  Total number of PCG iterations (trustregion algorithm only)  
stepsize  Final displacement in x (mediumscale algorithm only)  
message  Exit message 
fminunc computes the output argument hessian as follows:
When using the mediumscale algorithm, the function computes a finitedifference approximation to the Hessian at x using
The gradient grad if you supply it
The objective function fun if you do not supply the gradient
When using the trustregion algorithm, the function uses
options.Hessian, if you supply it, to compute the Hessian at x
A finitedifference approximation to the Hessian at x, if you supply only the gradient
fminunc uses these optimization options. Some options apply to all algorithms, some are only relevant when you are using the trustregion algorithm, and others are only relevant when you are using the quasinewton algorithm. Use optimoptions to set or change options. See Optimization Options Reference for detailed information.
All fminunc algorithms use the following options:
Algorithm If you use optimset (not recommended), use LargeScale instead of Algorithm.  Choose the fminunc algorithm. Choices are 'quasinewton' and 'trustregion' (default). The trustregion algorithm requires you to provide the gradient (see the preceding description of fun), or else fminunc uses the 'quasinewton' algorithm. For information on choosing the algorithm, see Choosing the Algorithm. 
DerivativeCheck  Compare usersupplied derivatives (gradient of objective) to finitedifferencing derivatives. The choices are 'on' or the default 'off'. 
Diagnostics  Display diagnostic information about the function to be minimized or solved. The choices are 'on' or the default 'off'. 
DiffMaxChange  Maximum change in variables for finitedifference gradients (a positive scalar). The default is Inf. 
DiffMinChange  Minimum change in variables for finitedifference gradients (a positive scalar). The default is 0. 
Display  Level of display:

FinDiffRelStep  Scalar or vector step size factor. When you set FinDiffRelStep to a vector v, forward finite differences delta are delta = v.*sign(x).*max(abs(x),TypicalX); and central finite differences are delta = v.*max(abs(x),TypicalX); Scalar FinDiffRelStep expands to a vector. The default is sqrt(eps) for forward finite differences, and eps^(1/3) for central finite differences. The trustregion algorithm uses FinDiffRelStep only when DerivativeCheck is 'on'. 
FinDiffType  Finite differences, used to estimate gradients, are either 'forward' (the default), or 'central' (centered). 'central' takes twice as many function evaluations, but should be more accurate. The trustregion algorithm uses FinDiffType only when DerivativeCheck is 'on'. 
FunValCheck  Check whether objective function values are valid. 'on' displays an error when the objective function returns a value that is complex, Inf, or NaN. The default, 'off', displays no error. 
GradObj  Gradient for the objective function defined by the user. See the preceding description of fun to see how to define the gradient in fun. Set to 'on' to have fminunc use a userdefined gradient of the objective function. The default 'off' causes fminunc to estimate gradients using finite differences. You must provide the gradient, and set GradObj to 'on', to use the trustregion algorithm. This option is not required for the quasiNewton algorithm. 
LargeScale If you use optimoptions (recommended), use Algorithm instead of LargeScale.  Use largescale algorithm if possible when set to the default 'on'. Use mediumscale algorithm when set to 'off'. The LargeScale algorithm requires you to provide the gradient (see the preceding description of fun), or else fminunc uses the mediumscale algorithm. For information on choosing the algorithm, see Choosing the Algorithm. 
MaxFunEvals  Maximum number of function evaluations allowed, a positive integer. The default value is 100*numberOfVariables. 
MaxIter  Maximum number of iterations allowed, a positive integer. The default value is 400. 
OutputFcn  Specify one or more userdefined functions that an optimization function calls at each iteration, either as a function handle or as a cell array of function handles. The default is none ([]). See Output Function. 
PlotFcns  Plots various measures of progress while the algorithm executes. Select from predefined plots or write your own. Pass a function handle or a cell array of function handles. The default is none ([]).
For information on writing a custom plot function, see Plot Functions. 
TolFun  Termination tolerance on the function value, a positive scalar. The default is 1e6. 
TolX  Termination tolerance on x, a positive scalar. The default value is 1e6. 
TypicalX  Typical x values. The number of elements in TypicalX is equal to the number of elements in x0, the starting point. The default value is ones(numberofvariables,1). fminunc uses TypicalX for scaling finite differences for gradient estimation. The trustregion algorithm uses TypicalX only for the DerivativeCheck option. 
The trustregion algorithm uses the following options:
Hessian  If 'on', fminunc uses a userdefined Hessian for the objective function. The Hessian is either defined in the objective function (see fun), or is indirectly defined when using HessMult. If 'off' (default), fminunc approximates the Hessian using finite differences.  
HessMult  Function handle for Hessian multiply function. For largescale structured problems, this function computes the Hessian matrix product H*Y without actually forming H. The function is of the form W = hmfun(Hinfo,Y) where Hinfo contains the matrix used to compute H*Y. The first argument must be the same as the third argument returned by the objective function fun, for example by [f,g,Hinfo] = fun(x) Y is a matrix that has the same number of rows as there are dimensions in the problem. W = H*Y although H is not formed explicitly. fminunc uses Hinfo to compute the preconditioner. See Passing Extra Parameters for information on how to supply values for any additional parameters hmfun needs. See Minimization with Dense Structured Hessian, Linear Equalities for an example.  
HessPattern  Sparsity pattern of the Hessian for finite differencing. Set HessPattern(i,j) = 1 when you can have ∂^{2}fun/∂x(i)∂x(j) ≠ 0. Otherwise, set HessPattern(i,j) = 0. Use HessPattern when it is inconvenient to compute the Hessian matrix H in fun, but you can determine (say, by inspection) when the ith component of the gradient of fun depends on x(j). fminunc can approximate H via sparse finite differences (of the gradient) if you provide the sparsity structure of H — i.e., locations of the nonzeros — as the value for HessPattern. In the worst case, when the structure is unknown, do not set HessPattern. The default behavior is as if HessPattern is a dense matrix of ones. Then fminunc computes a full finitedifference approximation in each iteration. This can be very expensive for large problems, so it is usually better to determine the sparsity structure.  
MaxPCGIter  Maximum number of PCG (preconditioned conjugate gradient) iterations, a positive scalar. The default is max(1,floor(numberOfVariables/2)). For more information, see Algorithms.  
PrecondBandWidth  Upper bandwidth of preconditioner for PCG, a nonnegative integer. By default, fminunc uses diagonal preconditioning (upper bandwidth of 0). For some problems, increasing the bandwidth reduces the number of PCG iterations. Setting PrecondBandWidth to Inf uses a direct factorization (Cholesky) rather than the conjugate gradients (CG). The direct factorization is computationally more expensive than CG, but produces a better quality step towards the solution.  
TolPCG  Termination tolerance on the PCG iteration, a positive scalar. The default is 0.1. 
The quasinewton algorithm uses the following options:
HessUpdate  Method for choosing the search direction in the QuasiNewton algorithm. The choices are:

InitialHessMatrix  Initial quasiNewton matrix. This option is only available if you set InitialHessType to 'usersupplied'. In that case, you can set InitialHessMatrix to one of the following:

InitialHessType  Initial quasiNewton matrix type. The options are:

ObjectiveLimit  A tolerance (stopping criterion) that is a scalar. If the objective function value at an iteration is less than or equal to ObjectiveLimit, the iterations halt, since the problem is presumably unbounded. The default value is 1e20. 
Minimize the function .
Create a file myfun.m:
function f = myfun(x) f = 3*x(1)^2 + 2*x(1)*x(2) + x(2)^2; % Cost function
Then call fminunc to find a minimum of myfun near [1,1]:
x0 = [1,1]; [x,fval] = fminunc(@myfun,x0);
After a few iterations, fminunc returns the solution, x, and the value of the function at x, fval:
x,fval x = 1.0e006 * 0.2541 0.2029 fval = 1.3173e013
To minimize this function with the gradient provided, modify myfun.m so the gradient is the second output argument:
function [f,g] = myfun(x) f = 3*x(1)^2 + 2*x(1)*x(2) + x(2)^2; % Cost function if nargout > 1 g(1) = 6*x(1)+2*x(2); g(2) = 2*x(1)+2*x(2); end
Indicate that the gradient value is available by creating optimization options with the GradObj option set to 'on' using optimoptions.
options = optimoptions('fminunc','GradObj','on'); x0 = [1,1]; [x,fval] = fminunc(@myfun,x0,options);
After several iterations fminunc returns the solution, x, and the value of the function at x, fval:
x,fval x = 1.0e015 * 0.1110 0.8882 fval = 6.2862e031
To minimize the function f(x) = sin(x) + 3 using an anonymous function
f = @(x)sin(x)+3; x = fminunc(f,4);
fminunc returns a solution
x x = 4.7124
fminunc is not the preferred choice for solving problems that are sums of squares, that is, of the form
Instead use the lsqnonlin function, which has been optimized for problems of this form.
To use the trustregion method, you must provide the gradient in fun (and set the GradObj option to 'on' using optimoptions). A warning is given if no gradient is provided and the Algorithm option is 'trustregion'.
The function to be minimized must be continuous. fminunc might only give local solutions.
fminunc only minimizes over the real numbers, that is, x must only consist of real numbers and f(x) must only return real numbers. When x has complex variables, they must be split into real and imaginary parts.
To use the trustregion algorithm, you must supply the gradient in fun (and GradObj must be set 'on' in options).
Trust Region Algorithm Coverage and Requirements
Additional Information Needed  For Large Problems 

Must provide gradient for f(x) in fun. 

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