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    1   /*
    2    * Copyright 2004 The Apache Software Foundation.
    3    *
    4    * Licensed under the Apache License, Version 2.0 (the "License");
    5    * you may not use this file except in compliance with the License.
    6    * You may obtain a copy of the License at
    7    *
    8    *      http://www.apache.org/licenses/LICENSE-2.0
    9    *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS,
    12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   * See the License for the specific language governing permissions and
    14   * limitations under the License.
    15   */
    16  package org.apache.commons.math.special;
    17
    18  import java.io.Serializable;
    19
    20  import org.apache.commons.math.MathException;
    21
    22  /***
    23   * This is a utility class that provides computation methods related to the
    24   * error functions.
    25   *
    26   * @version $Revision: 1.2 $ $Date: 2004/06/23 16:26:17 $
    27   */
    28  public class Erf implements Serializable {
    29
    30      /***
    31       * Default constructor.  Prohibit instantiation.
    32       */
    33      private Erf() {
    34          super();
    35      }
    36
    37      /***
    38       * Returns the error function erf(x).
    39       *
    40       * The implementation of this method is based on:
    41       * <ul>
    42       * <li>
    43       * <a href="http://mathworld.wolfram.com/Erf.html">
    44       * Erf</a>, equation (3).</li>
    45       * </ul>
    46       *
    47       * @param x the value.
    48       * @return the error function erf(x)
    49       * @throws MathException if the algorithm fails to converge.
    50       */
    51      public static double erf(double x) throws MathException {
    52          double ret = Gamma.regularizedGammaP(0.5, x * x, 1.0e-15, 10000);
    53          if (x < 0) {
    54              ret = -ret;
    55          }
    56          return ret;
    57      }
    58  }