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diff --git a/src/er/er_A.c b/src/er/er_A.c
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+/**
+ *  This program is free software: you can redistribute it and/or
+ *  modify it under the terms of the GNU General Public License as
+ *  published by the Free Software Foundation, either version 3 of the
+ *  License, or (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  (c) Vincenzo Nicosia 2009-2017 -- <v.nicosia@qmul.ac.uk>
+ * 
+ *  This file is part of NetBunch, a package for complex network
+ *  analysis and modelling. For more information please visit:
+ *
+ *             http://www.complex-networks.net/
+ *
+ *  If you use this software, please add a reference to 
+ *
+ *               V. Latora, V. Nicosia, G. Russo             
+ *   "Complex Networks: Principles, Methods and Applications"
+ *              Cambridge University Press (2017) 
+ *                ISBN: 9781107103184
+ *
+ ***********************************************************************
+ *
+ *  This program samples a graph from the Erdos-Renyi random graph
+ *  model A, i.e., the ensemble of graphs with N nodes and K edges.
+ *
+ *
+ *  References: 
+ *
+ *  [1] Erdős, P. & Rényi, A. On Random Graphs I Publ. Math. Debrecen,
+ *      6, 290 (1959)
+ *
+ *  [2] Erdos, P. & Renyi, A. On the evolution of random graphs
+ *      Publ. Math. Inst. Hungary. Acad. Sci., 5, 17-61 (1960)
+ *
+ */
+
+
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+#include "iltree_double.h"
+
+  
+
+void usage(char *argv[]){
+  printf("********************************************************************\n"
+         "**                                                                **\n"
+         "**                        -*-     er     -*-                      **\n"
+         "**                                                                **\n"
+         "**       sample an Erdos-Renyi graph of N nodes and K links       **\n"
+         "**                                                                **\n"
+         "**                  the output is an I,J matrix:                  **\n"
+         "**                            I_1 J_1                             **\n"
+         "**                            I_2 J_2                             **\n"
+         "**                            I_3 J_3                             **\n"
+         "**                            ... ...                             **\n"
+         "**                            I_K J_K                             **\n"
+         "**                                                                **\n"
+         "**               (output is to stdout or to a file)               **\n"
+         "**                                                                **\n"
+         "**      If a filename is specified as fourth parameter then       **\n"
+         "**         the degree sequence is dumped out to that file         **\n"
+         "**                                                                **\n"
+         "**                                                                **\n"
+         "********************************************************************\n"
+         " This is Free Software - You can use and distribute it under \n"
+         " the terms of the GNU General Public License, version 3 or later\n\n"
+         " (c) Vincenzo Nicosia 2010-2017 (v.nicosia@qmul.ac.uk)\n\n"
+         "********************************************************************\n\n"
+         );
+  printf("Usage: %s <N> <K> [<filout>]\n\n" , argv[0]);
+}
+
+
+void sample_edges(unsigned int N, double K, iltree_t t){
+  
+  long double k;
+  double i, k1, k2, tmp;
+  
+  for(i=0; i<K; ){
+    k1 = rand() % N +1; /* sample the first node uniformly in [0,N-1] */
+    k2 = rand() % N +1; /* sample the first node uniformly in [0,N-1] */
+    while (k1 == k2){ /* Avoid selfloops */
+      k1 = rand() % N +1; /* sample the first node uniformly in [0,N-1] */
+      k2 = rand() % N +1; /* sample the first node uniformly in [0,N-1] */
+    }
+    if (k1 > k2){
+      tmp = k2;
+      k2 = k1;
+      k1 = tmp;
+    }
+    k = 1.0 * (k2-1)* (k2-2) / 2 + k1; /* This number represents the couple k1, k2 (k1 < k2)*/
+    if (!iltree_lookup(t, &k)){ /* The element "k" is not yet present in the BST*/
+      /* So we insert into in the tree */
+      iltree_insert(t, &k);
+      /* and then we increment the counter of inserted edges */
+      i += 1;
+    }
+  }
+}
+
+
+
+int main(int argc, char *argv[]){
+
+  iltree_t t = NULL; /* This is the insert-lookup BST */
+  FILE *fileout;
+  char error_str [256];
+  unsigned int N;
+  double K;
+  
+  if(argc < 3){
+    usage(argv);
+    exit(1);
+  }
+
+  N = atoi(argv[1]);
+  K = atof(argv[2]);
+  
+  
+  if (argc >3){
+    fileout = fopen(argv[3], "w+");
+    if (!fileout){
+      sprintf(error_str, "Error opening file %s", argv[3]);
+      perror(error_str);
+      exit(2);
+    }
+  }
+  else{
+    fileout = stdout;
+  }
+  
+  t = iltree_double_init(t, fileout);
+
+  srand(time(NULL));
+  /* Now we sample K integers without repetition (using the BST)*/
+  sample_edges(N, K, t);
+  /* And then we dump out the edges in I,J format */
+  iltree_view_pre(t);
+  iltree_destroy(t);
+}