code: def lambda_tilde(mass1, mass2, lambda1, lambda2):
      
      m1, m2, lambda1, lambda2, input_is_array = ensurearray(
          mass1, mass2, lambda1, lambda2)
      lsum = lambda1 + lambda2
      ldiff, _ = ensurearray(lambda1 - lambda2)
      mask = m1 < m2
      ldiff[mask] = -ldiff[mask]
      eta = eta_from_mass1_mass2(m1, m2)
      p1 = (lsum) * (1 + 7. * eta - 31 * eta ** 2.0)
      p2 = (1 - 4 * eta)**0.5 * (1 + 9 * eta - 11 * eta ** 2.0) * (ldiff)
      return formatreturn(8.0 / 13.0 * (p1 + p2), input_is_array)
docs: The effective lambda parameter
  
      The mass-weighted dominant effective lambda parameter defined in
      https://journals.aps.org/prd/pdf/10.1103/PhysRevD.91.043002