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 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495  \chapter{Spendley's et al. method} \section{Analysis} The simplex algorithms are based on the iterative update of a \emph{simplex} made of $n+1$ points $S={x_i}_{i=1,n+1}$. Each point in the simplex is called a \emph{vertex} and is associated with a function value $f_i=f(x_i), i=1,n+1$. The vertices are sorted by increasing function values so that the \emph{best} vertex has index 1 and the \emph{worst} vertex has index $n+1$ \begin{eqnarray} \label{sorted-vertices-fv} f_1 \leq f_2 \leq \ldots \leq f_n \leq f_{n+1}. \end{eqnarray} The \emph{next-to-worst} vertex with index $n$ has a special role in simplex algorithms. The centroid of the simplex is the center of the vertices where the vertex with index $j=1,n+1$ has been excluded \begin{eqnarray} \label{centroid-generalized} \overline{x} (j) = \frac{1}{n} \sum_{i=1,n+1, i\neq j}x_i \end{eqnarray} The first move of the algorithm is based on the centroid where the worst vertex with index $j=n+1$ has been excluded \begin{eqnarray} \label{centroid-worst} \overline{x} (n+1) = \frac{1}{n} \sum_{i=1,n}x_i \end{eqnarray} The algorithm attemps to replace one vertex $x_j$ by a new point $x(\mu,j)$ between the centroid $\overline{x}$ and the vertex $x_j$ and defined by \begin{eqnarray} \label{interpolate-generalized} x(\mu,j) = (1+\mu)\overline{x}(j) - \mu x_j \end{eqnarray} The Spendley et al. \cite{Spendley1962} algorithm makes use of one coefficient, the reflection $\rho>0$. The standard value of this coefficient is $\rho=1$. The first move of the algorithm is based on the reflection with respect to the worst point $x_{n+1}$ so that the reflection point is computed by \begin{eqnarray} \label{interpolate-worst} x(\rho,n+1) = (1+\rho)\overline{x}(n+1) - \rho x_{n+1} \end{eqnarray} The algorithm first computes the reflection point with respect to the worst point excluded with $x_r=x(\rho,n+1)$ and evaluates the function value of the reflection point $f_r=f(x_r)$. If that value $f_r$ is better than the worst function value $f_{n+1}$, the worst point $x_{n+1}$ is rejected from the simplex and the reflection point $x_r$ is accepted. If the reflection point does not improves, the next-to-worst point $x_n$ is reflected and the function is evaluated at the new reflected point. If the function value improves over the worst function value $f_{n+1}$, the new reflection point is accepted. At that point of the algorithm, neither the reflection with respect to $x_{n+1}$ nor the reflection with respect to $x_n$ has improved. The algorithm therefore shrinks the simplex toward the best point. That last step uses the shrink coefficient $0<\sigma<1$. The standard value for this coefficient is $\sigma=\frac{1}{2}$. Spendley's et al. algorithm is presented in figure \ref{algo-spendley}. The figure \ref{fig-spendley-moves} presents the various moves of the Spendley et al. algorithm. It is obvious from the picture that the algorithm explores a pattern which is entirely determined from the initial simplex. \begin{figure}[htbp] \begin{algorithmic} \STATE Compute an initial simplex $S_0$ \STATE Sorts the vertices $S_0$ with increasing function values \STATE $S\gets S_0$ \WHILE{$\sigma(S)>tol$} \STATE $\overline{x}\gets \overline{x}(n+1)$ \STATE $x_r \gets x(\rho,n+1)$, $f_r \gets f(x_r)$ \COMMENT{Reflect with respect to worst} \IF {$f_r0$ is a chosen scaling parameter. The more $a$ is large, the more difficult the problem is to solve with the simplex algorithm. We set the maximum number of function evaluations to 400. The initial simplex is a regular simplex with length unity. The numerical results are presented in table \ref{fig-spendley-numexp1-table}, where the experiment is presented for $a=100$. One can check that the number of function evaluation is equal to its maximum limit, even if the value of the function at optimum is very inacurate ($f(x^\star) \approx 0.08$). \begin{figure}[h] \begin{center} \begin{tiny} \begin{tabular}{|l|l|} \hline Iterations & 340 \\ Function Evaluations & 400 \\ $a$ & $100.0$ \\ $x_0$ & $(10.0,10.0)$ \\ Relative tolerance on simplex size & $10^{-8}$ \\ Exact $x^\star$ & $(0.,0.)$\\ Computed $x^\star$ & $(0.001,0.2)$\\ Computed $f(x^\star)$ & $0.08$\\ \hline \end{tabular} \end{tiny} \end{center} \caption{Numerical experiment with Spendley's et al. method on a badly scaled quadratic function} \label{fig-spendley-numexp2-table} \end{figure} The various simplices generated during the iterations are presented in figure \ref{fig-spendley-numexp2-historysimplex}. The method use reflections in the early iterations. Then there is no possible improvment using reflections and shrinking is necessary. But the shrinking makes the simplex very small so that a large number of iterations are necessary to improve the function value. This is a limitation of the method, which is based on a simplex which can vary its size, but not its shape. \begin{figure} \begin{center} \includegraphics[width=10cm]{quad2-spendley-simplexcontours.png} \end{center} \caption{Spendley et al. numerical experiment with $f(x_1,x_2) = (a * x_1)^2 + x_2^2$ and $a=100$ -- history of simplex} \label{fig-spendley-numexp2-historysimplex} \end{figure} In figure \ref{fig-spendley-numexp2-scaling}, we analyse the behaviour of the method with respect to scaling. We check that the method behave poorly when the scaling is bad. The convergence speed is slower and slower and impractical when $a>10$ \begin{figure}[htbp] \begin{center} \begin{tiny} \begin{tabular}{|l|l|l|} \hline $a$ & Function evaluations & Computed $f(x^\star)$ \\ $1.0$ & 160 & $2.35e-18$ \\ $10.0$ & 222 & $1.2e-17$ \\ $100.0$ & 400 & $0.083$ \\ $1000.0$ & 400 & $30.3$ \\ $10000.0$ & 400 & $56.08$ \\ \hline \end{tabular} \end{tiny} \end{center} \caption{Numerical experiment with Spendley's et al. method on a badly scaled quadratic function} \label{fig-spendley-numexp2-scaling} \end{figure} \subsection{Sensitivity to dimension} In this section, we try to study the convergence of the Spendley et al. algorithm with respect to the number of variables. The function we try to minimize is the following quadratic in n-dimensions \begin{eqnarray} \label{quadratic-sp-function3} f(x_1,x_2) = \sum_{i=1,n} x_i^2. \end{eqnarray} The initial simplex is a regular simplex with length unity. The initial guess is at 0 so that this vertex is never updated during the iterations. For this test, we compute the rate of convergence as presented in Han \& Neuman. This rate is defined as \begin{eqnarray} \label{rho-sp-rate-convergence} \rho(S_0,n) = \textrm{lim sup}_{k\rightarrow \infty} \left(\sum_{i=0,k-1} \frac{\sigma(S_{i+1}}{\sigma(S_i}\right)^{1/k} \end{eqnarray} That definition can be viewed as the geometric mean of the ratio of the oriented lengths between successive simplices and the minimizer 0. This definition implies \begin{eqnarray} \label{rho-sp-rate-convergence2} \rho(S_0,n) = \textrm{lim sup}_{k\rightarrow \infty} \left( \frac{\sigma(S_{k+1}}{\sigma(S_0}\right)^{1/k} \end{eqnarray} The figure \ref{fig-sp-numexp3-dimension} presents the results of this experiment for $n=1,20$. The number and kids of performed steps are presented in figure \ref{fig-sp-numexp3-nbsteps}. It must be noticed that reflection step occurs rarely during the iterations : the algorithm mostly performs shrink steps. \begin{figure}[htbp] \begin{center} \begin{tiny} \begin{tabular}{|l|l|l|l|} \hline $n$ & \# Reflections & \# Reflection & \#Shrink\\ & / High & / Next to High & \\ \hline 1 & 0 & 0 & 27\\ 2 & 0 & 0 & 27\\ 3 & 1 & 0 & 27\\ 4 & 5 & 1 & 27\\ 5 & 0 & 0 & 27\\ 6 & 6 & 0 & 27\\ 7 & 4 & 0 & 27\\ 8 & 0 & 0 & 27\\ 9 & 12 & 1 & 27\\ 10 & 0 & 0 & 27\\ 11 & 0 & 0 & 27\\ 12 & 14 & 0 & 27\\ 13 & 0 & 0 & 27\\ 14 & 24 & 3 & 27\\ 15 & 0 & 0 & 27\\ 16 & 0 & 0 & 27\\ 17 & 21 & 0 & 27\\ 18 & 0 & 0 & 27\\ 19 & 28 & 0 & 27\\ \hline \end{tabular} \end{tiny} \end{center} \caption{Numerical experiment with Spendley et al method on a generalized quadratic function -- number and kinds of steps performed} \label{fig-sp-numexp3-nbsteps} \end{figure} One can check that the number of function evaluations increases approximately linearily with the dimension of the problem in figure \ref{fig-sp-numexp3-fvn}. A rough rule of thumb is that, for $n=1,19$, the number of function evaluations is equal to $30n$. This test is in fact the best that we can expect from this algorithm : since most iterations are shrink steps, most iterations improves the function value. \begin{figure}[htbp] \begin{center} \begin{tiny} \begin{tabular}{|l|l|l|l|} \hline $n$ & Function evaluations & Iterations & $\rho(S_0,n)$\\ \hline 1 & 83 & 28 & 0.5125321059829373\\ 2 & 111 & 28 & 0.5125321059829373\\ 3 & 140 & 29 & 0.52448212766151725\\ 4 & 174 & 34 & 0.57669577295965202\\ 5 & 195 & 28 & 0.5125321059829373\\ 6 & 229 & 34 & 0.57669577295965202\\ 7 & 255 & 32 & 0.55719337129794622\\ 8 & 279 & 28 & 0.5125321059829373\\ 9 & 321 & 41 & 0.63352059021162177\\ 10 & 335 & 28 & 0.5125321059829373\\ 11 & 363 & 28 & 0.5125321059829373\\ 12 & 405 & 42 & 0.64044334488213628\\ 13 & 419 & 28 & 0.5125321059829373\\ 14 & 477 & 55 & 0.71157656804932146\\ 15 & 475 & 28 & 0.5125321059829373\\ 16 & 503 & 28 & 0.5125321059829373\\ 17 & 552 & 49 & 0.68253720379799854\\ 18 & 559 & 28 & 0.5125321059829373\\ 19 & 615 & 56 & 0.71591347660379834\\ \hline \end{tabular} \end{tiny} \end{center} \caption{Numerical experiment with Spendley et al. method on a generalized quadratic function} \label{fig-sp-numexp3-dimension} \end{figure} \begin{figure} \begin{center} \includegraphics[width=10cm]{spendley-dimension-nfevals.png} \end{center} \caption{Spendley et al. numerical experiment -- number of function evaluations depending on the number of variables} \label{fig-sp-numexp3-fvn} \end{figure} The figure \ref{fig-nm-numexp3-rho} presents the rate of convergence depending on the number of variables. The figure shows that the rate of convergence rapidly gets close to 1 when the number of variables increases. That shows that the rate of convergence is slower and slower as the number of variables increases, as explained by Han \& Neuman. \begin{figure} \begin{center} \includegraphics[width=10cm]{spendley-dimension-rho.png} \end{center} \caption{Spendley et al. numerical experiment -- rate of convergence depending on the number of variables} \label{fig-sp-numexp3-rho} \end{figure} \section{Conclusion} We saw in the first numerical experiment that the method behave reasonably when the function is correctly scaled. When the function is badly scaled, as in the second numerical experiment, the Spendley et al. algorithm produces a large number of function evaluations and converges very slowly. This limitation occurs with even moderate badly scaled functions and generates a very slow method in these cases.