2.7. Mathematical optimization: finding minima of functions¶
Authors: Gaël Varoquaux
Mathematical optimization deals with the problem of finding numerically minimums (or maximums or zeros) of a function. In this context, the function is called cost function, or objective function, or energy.
Here, we are interested in using scipy.optimize
for blackbox optimization: we do not rely on the mathematical expression of the function that we are optimizing. Note that this expression can often be used for more efficient, non blackbox, optimization.
Prerequisites
 Numpy, Scipy
 matplotlib
See also
References
Mathematical optimization is very ... mathematical. If you want performance, it really pays to read the books:
 Convex Optimization by Boyd and Vandenberghe (pdf available free online).
 Numerical Optimization, by Nocedal and Wright. Detailed reference on gradient descent methods.
 Practical Methods of Optimization by Fletcher: good at handwaving explanations.
Chapters contents
2.7.1. Knowing your problem¶
Not all optimization problems are equal. Knowing your problem enables you to choose the right tool.
Dimensionality of the problem
The scale of an optimization problem is pretty much set by the dimensionality of the problem, i.e. the number of scalar variables on which the search is performed.
2.7.1.1. Convex versus nonconvex optimization¶
A convex function:

A nonconvex function 
Optimizing convex functions is easy. Optimizing nonconvex functions can be very hard.
Note
It can be proven that for a convex function a local minimum is also a global minimum. Then, in some sense, the minimum is unique.
2.7.1.2. Smooth and nonsmooth problems¶
A smooth function: The gradient is defined everywhere, and is a continuous function 
A nonsmooth function 
Optimizing smooth functions is easier (true in the context of blackbox optimization, otherwise Linear Programming is an example of methods which deal very efficiently with piecewise linear functions).
2.7.1.3. Noisy versus exact cost functions¶
Noisy (blue) and nonnoisy (green) functions 
Noisy gradients
Many optimization methods rely on gradients of the objective function. If the gradient function is not given, they are computed numerically, which induces errors. In such situation, even if the objective function is not noisy, a gradientbased optimization may be a noisy optimization.
2.7.1.4. Constraints¶
Optimizations under constraints Here: 
2.7.2. A review of the different optimizers¶
2.7.2.1. Getting started: 1D optimization¶
Use scipy.optimize.brent()
to minimize 1D functions. It combines a bracketing strategy with a parabolic approximation.
Brent’s method on a quadratic function: it converges in 3 iterations, as the quadratic approximation is then exact.  
Brent’s method on a nonconvex function: note that the fact that the optimizer avoided the local minimum is a matter of luck. 
>>> from scipy import optimize
>>> def f(x):
... return np.exp((x  .7)**2)
>>> x_min = optimize.brent(f) # It actually converges in 9 iterations!
>>> x_min
0.699999999...
>>> x_min  .7
2.1605...e10
Note
Brent’s method can also be used for optimization constrained to an interval using scipy.optimize.fminbound()
Note
In scipy 0.11, scipy.optimize.minimize_scalar()
gives a generic interface to 1D scalar minimization
2.7.2.2. Gradient based methods¶
2.7.2.2.1. Some intuitions about gradient descent¶
Here we focus on intuitions, not code. Code will follow.
Gradient descent basically consists in taking small steps in the direction of the gradient, that is the direction of the steepest descent.
A wellconditioned quadratic function.  
An illconditioned quadratic function. The core problem of gradientmethods on illconditioned problems is that the gradient tends not to point in the direction of the minimum. 
We can see that very anisotropic (illconditioned) functions are harder to optimize.
Take home message: conditioning number and preconditioning
If you know natural scaling for your variables, prescale them so that they behave similarly. This is related to preconditioning.
Also, it clearly can be advantageous to take bigger steps. This is done in gradient descent code using a line search.
A wellconditioned quadratic function.  
An illconditioned quadratic function.  
An illconditioned nonquadratic function.  
An illconditioned very nonquadratic function. 
The more a function looks like a quadratic function (elliptic isocurves), the easier it is to optimize.
2.7.2.2.2. Conjugate gradient descent¶
The gradient descent algorithms above are toys not to be used on real problems.
As can be seen from the above experiments, one of the problems of the simple gradient descent algorithms, is that it tends to oscillate across a valley, each time following the direction of the gradient, that makes it cross the valley. The conjugate gradient solves this problem by adding a friction term: each step depends on the two last values of the gradient and sharp turns are reduced.
An illconditioned nonquadratic function.  
An illconditioned very nonquadratic function. 
Methods based on conjugate gradient are named with ‘cg’ in scipy. The simple conjugate gradient method to minimize a function is scipy.optimize.fmin_cg()
:
>>> def f(x): # The rosenbrock function
... return .5*(1  x[0])**2 + (x[1]  x[0]**2)**2
>>> optimize.fmin_cg(f, [2, 2])
Optimization terminated successfully.
Current function value: 0.000000
Iterations: 13
Function evaluations: 120
Gradient evaluations: 30
array([ 0.99998968, 0.99997855])
These methods need the gradient of the function. They can compute it, but will perform better if you can pass them the gradient:
>>> def fprime(x):
... return np.array((2*.5*(1  x[0])  4*x[0]*(x[1]  x[0]**2), 2*(x[1]  x[0]**2)))
>>> optimize.fmin_cg(f, [2, 2], fprime=fprime)
Optimization terminated successfully.
Current function value: 0.000000
Iterations: 13
Function evaluations: 30
Gradient evaluations: 30
array([ 0.99999199, 0.99998336])
Note that the function has only been evaluated 30 times, compared to 120 without the gradient.
2.7.2.3. Newton and quasinewton methods¶
2.7.2.3.1. Newton methods: using the Hessian (2nd differential)¶
Newton methods use a local quadratic approximation to compute the jump direction. For this purpose, they rely on the 2 first derivative of the function: the gradient and the Hessian.
An illconditioned quadratic function: Note that, as the quadratic approximation is exact, the Newton method is blazing fast 

An illconditioned nonquadratic function: Here we are optimizing a Gaussian, which is always below its quadratic approximation. As a result, the Newton method overshoots and leads to oscillations. 

An illconditioned very nonquadratic function: 
In scipy, the Newton method for optimization is implemented in scipy.optimize.fmin_ncg()
(cg here refers to that fact that an inner operation, the inversion of the Hessian, is performed by conjugate gradient). scipy.optimize.fmin_tnc()
can be use for constraint problems, although it is less versatile:
>>> def f(x): # The rosenbrock function
... return .5*(1  x[0])**2 + (x[1]  x[0]**2)**2
>>> def fprime(x):
... return np.array((2*.5*(1  x[0])  4*x[0]*(x[1]  x[0]**2), 2*(x[1]  x[0]**2)))
>>> optimize.fmin_ncg(f, [2, 2], fprime=fprime)
Optimization terminated successfully.
Current function value: 0.000000
Iterations: 9
Function evaluations: 11
Gradient evaluations: 51
Hessian evaluations: 0
array([ 1., 1.])
Note that compared to a conjugate gradient (above), Newton’s method has required less function evaluations, but more gradient evaluations, as it uses it to approximate the Hessian. Let’s compute the Hessian and pass it to the algorithm:
>>> def hessian(x): # Computed with sympy
... return np.array(((1  4*x[1] + 12*x[0]**2, 4*x[0]), (4*x[0], 2)))
>>> optimize.fmin_ncg(f, [2, 2], fprime=fprime, fhess=hessian)
Optimization terminated successfully.
Current function value: 0.000000
Iterations: 9
Function evaluations: 11
Gradient evaluations: 19
Hessian evaluations: 9
array([ 1., 1.])
Note
At very highdimension, the inversion of the Hessian can be costly and unstable (large scale > 250).
Note
Newton optimizers should not to be confused with Newton’s root finding method, based on the same principles, scipy.optimize.newton()
.
2.7.2.3.2. QuasiNewton methods: approximating the Hessian on the fly¶
BFGS: BFGS (BroydenFletcherGoldfarbShanno algorithm) refines at each step an approximation of the Hessian.
An illconditioned quadratic function: On a exactly quadratic function, BFGS is not as fast as Newton’s method, but still very fast. 

An illconditioned nonquadratic function: Here BFGS does better than Newton, as its empirical estimate of the curvature is better than that given by the Hessian. 

An illconditioned very nonquadratic function: 
>>> def f(x): # The rosenbrock function
... return .5*(1  x[0])**2 + (x[1]  x[0]**2)**2
>>> def fprime(x):
... return np.array((2*.5*(1  x[0])  4*x[0]*(x[1]  x[0]**2), 2*(x[1]  x[0]**2)))
>>> optimize.fmin_bfgs(f, [2, 2], fprime=fprime)
Optimization terminated successfully.
Current function value: 0.000000
Iterations: 16
Function evaluations: 24
Gradient evaluations: 24
array([ 1.00000017, 1.00000026])
LBFGS: Limitedmemory BFGS Sits between BFGS and conjugate gradient: in very high dimensions (> 250) the Hessian matrix is too costly to compute and invert. LBFGS keeps a lowrank version. In addition, the scipy version, scipy.optimize.fmin_l_bfgs_b()
, includes box bounds:
>>> def f(x): # The rosenbrock function
... return .5*(1  x[0])**2 + (x[1]  x[0]**2)**2
>>> def fprime(x):
... return np.array((2*.5*(1  x[0])  4*x[0]*(x[1]  x[0]**2), 2*(x[1]  x[0]**2)))
>>> optimize.fmin_l_bfgs_b(f, [2, 2], fprime=fprime)
(array([ 1.00000005, 1.00000009]), 1.4417677473011859e15, {...})
Note
If you do not specify the gradient to the LBFGS solver, you need to add approx_grad=1
2.7.2.4. Gradientless methods¶
2.7.2.4.1. A shooting method: the Powell algorithm¶
Almost a gradient approach
An illconditioned quadratic function: Powell’s method isn’t too sensitive to local illconditionning in low dimensions 

An illconditioned very nonquadratic function: 
2.7.2.4.2. Simplex method: the NelderMead¶
The NelderMead algorithms is a generalization of dichotomy approaches to highdimensional spaces. The algorithm works by refining a simplex, the generalization of intervals and triangles to highdimensional spaces, to bracket the minimum.
Strong points: it is robust to noise, as it does not rely on computing gradients. Thus it can work on functions that are not locally smooth such as experimental data points, as long as they display a largescale bellshape behavior. However it is slower than gradientbased methods on smooth, nonnoisy functions.
An illconditioned nonquadratic function:  
An illconditioned very nonquadratic function: 
In scipy, scipy.optimize.fmin()
implements the NelderMead approach:
>>> def f(x): # The rosenbrock function
... return .5*(1  x[0])**2 + (x[1]  x[0]**2)**2
>>> optimize.fmin(f, [2, 2])
Optimization terminated successfully.
Current function value: 0.000000
Iterations: 46
Function evaluations: 91
array([ 0.99998568, 0.99996682])
2.7.2.5. Global optimizers¶
If your problem does not admit a unique local minimum (which can be hard to test unless the function is convex), and you do not have prior information to initialize the optimization close to the solution, you may need a global optimizer.
2.7.2.5.1. Brute force: a grid search¶
scipy.optimize.brute()
evaluates the function on a given grid of parameters and returns the parameters corresponding to the minimum value. The parameters are specified with ranges given to numpy.mgrid
. By default, 20 steps are taken in each direction:
>>> def f(x): # The rosenbrock function
... return .5*(1  x[0])**2 + (x[1]  x[0]**2)**2
>>> optimize.brute(f, ((1, 2), (1, 2)))
array([ 1.00001462, 1.00001547])
2.7.3. Practical guide to optimization with scipy¶
2.7.3.1. Choosing a method¶
Without knowledge of the gradient:  



With knowledge of the gradient:  


With the Hessian:  


If you have noisy measurements:  

2.7.3.2. Making your optimizer faster¶
 Choose the right method (see above), do compute analytically the gradient and Hessian, if you can.
 Use preconditionning when possible.
 Choose your initialization points wisely. For instance, if you are running many similar optimizations, warmrestart one with the results of another.
 Relax the tolerance if you don’t need precision
2.7.3.3. Computing gradients¶
Computing gradients, and even more Hessians, is very tedious but worth the effort. Symbolic computation with Sympy may come in handy.
Warning
A very common source of optimization not converging well is human error in the computation of the gradient. You can use scipy.optimize.check_grad()
to check that your gradient is correct. It returns the norm of the different between the gradient given, and a gradient computed numerically:
>>> optimize.check_grad(f, fprime, [2, 2])
2.384185791015625e07
See also scipy.optimize.approx_fprime()
to find your errors.
2.7.3.4. Synthetic exercices¶
Exercice: A simple (?) quadratic function
Optimize the following function, using K[0] as a starting point:
np.random.seed(0)
K = np.random.normal(size=(100, 100))
def f(x):
return np.sum((np.dot(K, x  1))**2) + np.sum(x**2)**2
Time your approach. Find the fastest approach. Why is BFGS not working well?
2.7.4. Special case: nonlinear leastsquares¶
2.7.4.1. Minimizing the norm of a vector function¶
Least square problems, minimizing the norm of a vector function, have a specific structure that can be used in the Levenberg–Marquardt algorithm implemented in scipy.optimize.leastsq()
.
Lets try to minimize the norm of the following vectorial function:
>>> def f(x):
... return np.arctan(x)  np.arctan(np.linspace(0, 1, len(x)))
>>> x0 = np.zeros(10)
>>> optimize.leastsq(f, x0)
(array([ 0. , 0.11111111, 0.22222222, 0.33333333, 0.44444444,
0.55555556, 0.66666667, 0.77777778, 0.88888889, 1. ]), 2)
This took 67 function evaluations (check it with ‘full_output=1’). What if we compute the norm ourselves and use a good generic optimizer (BFGS):
>>> def g(x):
... return np.sum(f(x)**2)
>>> optimize.fmin_bfgs(g, x0)
Optimization terminated successfully.
Current function value: 0.000000
Iterations: 11
Function evaluations: 144
Gradient evaluations: 12
array([ 7.4...09, 1.1...e01, 2.2...e01,
3.3...e01, 4.4...e01, 5.5...e01,
6.6...e01, 7.7...e01, 8.8...e01,
1.0...e+00])
BFGS needs more function calls, and gives a less precise result.
Note
leastsq is interesting compared to BFGS only if the dimensionality of the output vector is large, and larger than the number of parameters to optimize.
Warning
If the function is linear, this is a linearalgebra problem, and should be solved with scipy.linalg.lstsq()
.
2.7.4.2. Curve fitting¶
Least square problems occur often when fitting a nonlinear to data. While it is possible to construct our optimization problem ourselves, scipy provides a helper function for this purpose: scipy.optimize.curve_fit()
:
>>> def f(t, omega, phi):
... return np.cos(omega * t + phi)
>>> x = np.linspace(0, 3, 50)
>>> y = f(x, 1.5, 1) + .1*np.random.normal(size=50)
>>> optimize.curve_fit(f, x, y)
(array([ 1.51854577, 0.92665541]), array([[ 0.00037994, 0.00056796],
[0.00056796, 0.00123978]]))
Exercise
Do the same with omega = 3. What is the difficulty?
2.7.5. Optimization with constraints¶
2.7.5.1. Box bounds¶
Box bounds correspond to limiting each of the individual parameters of the optimization. Note that some problems that are not originally written as box bounds can be rewritten as such via change of variables.
scipy.optimize.fminbound()
for 1Doptimizationscipy.optimize.fmin_l_bfgs_b()
a quasiNewton method with bound constraints:>>> def f(x): ... return np.sqrt((x[0]  3)**2 + (x[1]  2)**2) >>> optimize.fmin_l_bfgs_b(f, np.array([0, 0]), approx_grad=1, bounds=((1.5, 1.5), (1.5, 1.5))) (array([ 1.5, 1.5]), 1.5811388300841898, {...})
2.7.5.2. General constraints¶
Equality and inequality constraints specified as functions: f(x) = 0 and g(x)< 0.
scipy.optimize.fmin_slsqp()
Sequential least square programming: equality and inequality constraints:>>> def f(x): ... return np.sqrt((x[0]  3)**2 + (x[1]  2)**2) >>> def constraint(x): ... return np.atleast_1d(1.5  np.sum(np.abs(x))) >>> optimize.fmin_slsqp(f, np.array([0, 0]), ieqcons=[constraint, ]) Optimization terminated successfully. (Exit mode 0) Current function value: 2.47487373504 Iterations: 5 Function evaluations: 20 Gradient evaluations: 5 array([ 1.25004696, 0.24995304])
scipy.optimize.fmin_cobyla()
Constraints optimization by linear approximation: inequality constraints only:>>> optimize.fmin_cobyla(f, np.array([0, 0]), cons=constraint) array([ 1.25009622, 0.24990378])
Warning
The above problem is known as the Lasso problem in statistics, and there exists very efficient solvers for it (for instance in scikitlearn). In general do not use generic solvers when specific ones exist.
Lagrange multipliers
If you are ready to do a bit of math, many constrained optimization problems can be converted to nonconstrained optimization problems using a mathematical trick known as Lagrange multipliers.