Virasoro algebra

Virasoro algebra is the central extension of a Lie algebra with wide applications in the two-dimensional conformal field theory. Its generators \(L_n, n \in \mathbb{Z}\) satisfy commutation relations

\[[L_m, L_n] = (m - n) L_{m+n} + c(m^3 - m) \delta_{m, -n},\]

where \(c\) is the central charge commuting with all generators. It was shown in [FNZ88] that the Virasoro algebra can be constructed out of just two generators \(L_3\) and \(L_{-2}\) using the following recurrence relations,

\[\begin{split}\begin{align} L_1 &= \frac{1}{5}[L_3, L_{-2}],\\ L_{-1} &= \frac{1}{3}[L_1, L_{-2}],\\ L_2 &= \frac{1}{4}[L_3, L_{-1}],\\ L_0 &= \frac{1}{2}[L_1, L_{-1}],\\ L_{n+1} &= \frac{1}{n-1}[L_n, L_1] \text{ for } n>2,\\ L_{-n-1} &= \frac{1}{1-n}[L_{-n}, L_{-1}] \text{ for } n>1. \end{align}\end{split}\]

In the example below, we show how to implement the Virasoro algebra in libcommute’s framework and use it to verify the recurrence relations stated above.

#include <iostream>
#include <memory>
#include <tuple>

#include <libcommute/libcommute.hpp>

namespace libcommute {

// Define a new generator type by deriving from the abstract base class
// 'libcommute::generator<IndexTypes...>'. Generators of the Virasoro algebra
// L_n carry one integer index 'n'.
// Canonical order of generators in a monomial L_n * L_m * L_k * ... is such
// that n < m < k < ...
class generator_virasoro : public generator<int> {

  using base = generator<int>;
  using linear_function_t = typename base::linear_function_t;

  // For the sake of simplicity, we fix the central charge once and for all.
  static constexpr double central_charge = 2.0;

public:
  // This function must return a unique algebra ID shared by all generators
  // of a particular algebra.
  int algebra_id() const override {
    // Use the lowest algebra ID available to user-defined algebras
    return min_user_defined_algebra_id;
  }

  // Construct generator with a given index 'n'
  explicit generator_virasoro(int n) : base(n) {}
  // Standard constructors and destructor
  generator_virasoro(generator_virasoro const&) = default;
  generator_virasoro(generator_virasoro&&) noexcept = default;
  ~generator_virasoro() override = default;

  // Generator objects must be immutable
  generator_virasoro& operator=(generator_virasoro const&) = delete;
  generator_virasoro& operator=(generator_virasoro&&) noexcept = delete;

  // Given a product L_m * L_n with m > n, transform it to the canonically
  // ordered form L_n * L_m plus some linear function of generators.
  // For the Virasoro algebra the transformation is
  //
  // L_m * L_n -> L_n * L_m + (m-n)*L_{m+n} + c(m^3 - m)\delta(m,-n)
  //
  // L_m will be passed to swap_with() as *this, i.e. L_m.swap_with(L_n, f).
  var_number swap_with(base const& L_n, linear_function_t& f) const override {
    // Ensure that L_m > L_n, or equivalently m > n.
    assert(*this > L_n);

    auto const& L_n_ = dynamic_cast<generator_virasoro const&>(L_n);

    // Extract indices from L_m and L_n
    int m = std::get<0>(base::indices());
    int n = std::get<0>(L_n_.indices());

    // Write linear terms of the transformed expressions into 'f'
    f.set(m == -n ? (central_charge * (m * m * m - m)) : 0, // Constant term
          std::make_shared<generator_virasoro>(m + n),      // L_{m+n}
          m - n // Coefficient in front of L_{m+n}
    );

    // Return coefficient in front of L_n * L_m in the transformed expression
    return 1;
  }

  // Given a product L_m * L_n with m <= n, optionally transform it to
  // some linear function of generators. For the Virasoro algebra such a
  // transformation exists for L_m = L_n,
  //
  // L_m * L_n -> 0
  //
  // L_m will be passed to simplify_prod() as *this,
  // i.e. L_m.simplify_prod(L_n, f).
  bool simplify_prod(base const& L_n, linear_function_t& f) const override {
    // Ensure that L_m <= L_n, or equivalently m <= n.
    assert(!(*this > L_n));

    if(*this == L_n) {
      // The transformed product is identically zero
      f.set(0);
      return true;
    } else
      return false; // No suitable transformation can be applied
  }

  // Hermitian conjugate: (L_n)^\dagger = L_{-n}
  void conj(linear_function_t& f) const override {
    int conj_n = -std::get<0>(base::indices());
    f.set(0, std::make_shared<generator_virasoro>(conj_n), 1);
  }

  // String representation of L_n
  std::string to_string() const override {
    int n = std::get<0>(base::indices());
    return "L(" + std::to_string(n) + ")";
  }
};

// Convenience factory function to create expressions made of one
// monomial L_n.
expression<double, int> L(int n) {
  using ret_t = expression<double, int>;
  return ret_t(1.0, ret_t::monomial_t(generator_virasoro(n)));
}

} // namespace libcommute

using namespace libcommute;

int main() {

  // Check that L(0) is Hermitian and (L_n)^\dagger = L_{-n}
  std::cout << "conj(L(0)) = " << conj(L(0)) << '\n';
  std::cout << "conj(L(1)) = " << conj(L(1)) << '\n';

  // Check that L(n)^2 = 0 for a few n
  std::cout << "L(0) * L(0) = " << L(0) * L(0) << '\n';
  std::cout << "L(1) * L(1) = " << L(1) * L(1) << '\n';
  std::cout << "L(-1) * L(-1) = " << L(-1) * L(-1) << '\n';

  // Check recurrence relations from Eq. (5)
  std::cout << "L_1 - (1/5)[L_3, L_{-2}] = "
            << (L(1) - (1.0 / 5) * (L(3) * L(-2) - L(-2) * L(3))) << '\n';
  std::cout << "L_{-1} - (1/3)[L_1, L_{-2}] = "
            << (L(-1) - (1.0 / 3) * (L(1) * L(-2) - L(-2) * L(1))) << '\n';
  std::cout << "L_2 - (1/4)[L_3, L_{-1}] = "
            << (L(2) - (1.0 / 4) * (L(3) * L(-1) - L(-1) * L(3))) << '\n';
  std::cout << "L_0 - (1/2)[L_1, L_{-1}] = "
            << (L(0) - (1.0 / 2) * (L(1) * L(-1) - L(-1) * L(1))) << '\n';

  // Check recurrence relation Eq. (6) for some higher positive n
  for(int n = 3; n < 10; ++n) {
    std::cout << "L_" << (n + 1) << " - (1/" << (n - 1) << ")"
              << "[L_" << n << ", L_1] = "
              << (L(n + 1) - (1.0 / (n - 1)) * (L(n) * L(1) - L(1) * L(n)))
              << '\n';
  }
  // Check recurrence relation Eq. (7) for some higher negative n
  for(int n = 2; n < 10; ++n) {
    std::cout << "L_" << (-n - 1) << " - (1/(" << (1 - n) << "))"
              << "[L_" << -n << ", L_{-1}] = "
              << (L(-n - 1) - (1.0 / (1 - n)) * (L(-n) * L(-1) - L(-1) * L(-n)))
              << '\n';
  }

  return 0;
}

[FNZ88]

“A presentation for the Virasoro and super-Virasoro algebras”, D. B. Fairlie, J. Nuyts and C. K. Zachos , Commun. Math. Phys. 117, pp. 595–614 (1988), https://doi.org/10.1007/BF01218387