optimize                package:stats                R Documentation

_O_n_e _D_i_m_e_n_s_i_o_n_a_l _O_p_t_i_m_i_z_a_t_i_o_n

_D_e_s_c_r_i_p_t_i_o_n:

     The function 'optimize' searches the interval from 'lower' to
     'upper' for a minimum or maximum of the function 'f' with respect
     to its first argument.

     'optimise' is an alias for 'optimize'.

_U_s_a_g_e:

     optimize(f = , interval = , lower = min(interval),
             upper = max(interval), maximum = FALSE,
             tol = .Machine$double.eps^0.25, ...)
     optimise(f = , interval = , lower = min(interval),
             upper = max(interval), maximum = FALSE,
             tol = .Machine$double.eps^0.25, ...)

_A_r_g_u_m_e_n_t_s:

       f: the function to be optimized. The function is either
          minimized or maximized over its first argument depending on
          the value of 'maximum'.

interval: a vector containing the end-points of the interval to be
          searched for the minimum.

   lower: the lower end point of the interval to be searched.

   upper: the upper end point of the interval to be searched.

 maximum: logical. Should we maximize or minimize (the default)?

     tol: the desired accuracy.

     ...: additional arguments to 'f'.

_D_e_t_a_i_l_s:

     The method used is a combination of golden section search and
     successive parabolic interpolation.  Convergence is never much
     slower than that for a Fibonacci search.  If 'f' has a continuous
     second derivative which is positive at the minimum (which is not
     at 'lower' or 'upper'), then convergence is superlinear, and
     usually of the order of about 1.324.

     The function 'f' is never evaluated at two points closer together
     than eps * |x_0| + (tol/3), where eps is approximately
     'sqrt(.Machine$double.eps)' and x_0 is the final abscissa
     'optimize()$minimum'.
      If 'f' is a unimodal function and the computed values of 'f' are
     always unimodal when separated by at least eps * |x| + (tol/3),
     then x_0 approximates the abscissa of the global minimum of 'f' on
     the interval 'lower,upper' with an error less than eps * |x_0|+
     tol.
      If 'f' is not unimodal, then 'optimize()' may approximate a
     local, but perhaps non-global, minimum to the same accuracy.

     The first evaluation of 'f' is always at x_1 = a + (1-phi)(b-a)
     where '(a,b) = (lower, upper)' and phi = (sqrt 5 - 1)/2 =
     0.61803.. is the golden section ratio. Almost always, the second
     evaluation is at x_2 = a + phi(b-a). Note that a local minimum
     inside [x_1,x_2] will be found as solution, even when 'f' is
     constant in there, see the last example.

     It uses a C translation of Fortran code (from Netlib) based on the
     Algol 60 procedure 'localmin' given in the reference.

_V_a_l_u_e:

     A list with components 'minimum' (or 'maximum') and 'objective'
     which give the location of the minimum (or maximum) and the value
     of the function at that point.

_R_e_f_e_r_e_n_c_e_s:

     Brent, R. (1973) _Algorithms for Minimization without
     Derivatives._ Englewood Cliffs N.J.: Prentice-Hall.

_S_e_e _A_l_s_o:

     'nlm', 'uniroot'.

_E_x_a_m_p_l_e_s:

     f <- function (x,a) (x-a)^2
     xmin <- optimize(f, c(0, 1), tol = 0.0001, a = 1/3)
     xmin

     ## See where the function is evaluated:
     optimize(function(x) x^2*(print(x)-1), l=0, u=10)

     ## "wrong" solution with unlucky interval and piecewise constant f():
     f  <- function(x) ifelse(x > -1, ifelse(x < 4, exp(-1/abs(x - 1)), 10), 10)
     fp <- function(x) { print(x); f(x) }

     plot(f, -2,5, ylim = 0:1, col = 2)
     optimize(fp, c(-4, 20))# doesn't see the minimum
     optimize(fp, c(-7, 20))# ok

