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diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/PL_approximation.h b/src/Coxeter_triangulation/include/gudhi/Functions/PL_approximation.h
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+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_PL_APPROXIMATION_H_
+#define FUNCTIONS_PL_APPROXIMATION_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class PL_approximation
+ * \brief Constructs a piecewise-linear approximation of a function induced by
+ * an ambient triangulation.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ * \tparam Triangulation The triangulation template parameter. Should be a model of
+ * the concept TriangulationForManifoldTracing.
+ */
+template <class Function_, class Triangulation_>
+struct PL_approximation {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    std::size_t cod_d = this->cod_d();
+    std::size_t amb_d = this->amb_d();
+    auto s = tr_.locate_point(p);
+    Eigen::MatrixXd matrix(cod_d, s.dimension() + 1);
+    Eigen::MatrixXd vertex_matrix(amb_d + 1, s.dimension() + 1);
+    for (std::size_t i = 0; i < s.dimension() + 1; ++i) vertex_matrix(0, i) = 1;
+    std::size_t j = 0;
+    for (auto v : s.vertex_range()) {
+      Eigen::VectorXd pt_v = tr_.cartesian_coordinates(v);
+      Eigen::VectorXd fun_v = fun_(pt_v);
+      for (std::size_t i = 1; i < amb_d + 1; ++i) vertex_matrix(i, j) = pt_v(i - 1);
+      for (std::size_t i = 0; i < cod_d; ++i) matrix(i, j) = fun_v(i);
+      j++;
+    }
+    assert(j == s.dimension() + 1);
+    Eigen::VectorXd z(amb_d + 1);
+    z(0) = 1;
+    for (std::size_t i = 1; i < amb_d + 1; ++i) z(i) = p(i - 1);
+    Eigen::VectorXd lambda = vertex_matrix.colPivHouseholderQr().solve(z);
+    Eigen::VectorXd result = matrix * lambda;
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return fun_.amb_d(); }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return fun_.cod_d(); }
+
+  /** \brief Returns a point on the zero-set. */
+  Eigen::VectorXd seed() const {
+    // TODO: not finished. Should use an oracle.
+    return Eigen::VectorXd(amb_d());
+  }
+
+  /**
+   * \brief Constructor of the piecewise-linear approximation of a function
+   * induced by an ambient triangulation.
+   *
+   * @param[in] function The function.
+   * @param[in] triangulation The ambient triangulation.
+   */
+  PL_approximation(const Function_& function, const Triangulation_& triangulation)
+      : fun_(function), tr_(triangulation) {}
+
+ private:
+  Function_ fun_;
+  Triangulation_ tr_;
+};
+
+/**
+ * \brief Static constructor of the piecewise-linear approximation of a function
+ * induced by an ambient triangulation.
+ *
+ * @param[in] function The function.
+ * @param[in] triangulation The ambient triangulation.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ *
+ * \ingroup coxeter_triangulation
+ */
+template <class Function_, class Triangulation_>
+PL_approximation<Function_, Triangulation_> make_pl_approximation(const Function_& function,
+                                                                  const Triangulation_& triangulation) {
+  return PL_approximation<Function_, Triangulation_>(function, triangulation);
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif