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diff --git a/src/main/java/org/apache/commons/math3/ml/neuralnet/sofm/NeighbourhoodSizeFunction.java b/src/main/java/org/apache/commons/math3/ml/neuralnet/sofm/NeighbourhoodSizeFunction.java
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.math3.ml.neuralnet.sofm;
+
+/**
+ * Provides the network neighbourhood's size as a function of the
+ * number of calls already performed during the learning task.
+ * The "neighbourhood" is the set of neurons that can be reached
+ * by traversing at most the number of links returned by this
+ * function.
+ *
+ * @since 3.3
+ */
+public interface NeighbourhoodSizeFunction {
+    /**
+     * Computes the neighbourhood size at the current call.
+     *
+     * @param numCall Current step of the training task.
+     * @return the value of the function at {@code numCall}.
+     */
+    int value(long numCall);
+}