Update EmittanceInvariants.H

Add literature citations.

src/particles/diagnostics/EmittanceInvariants.H

@@ -21,11 +21,18 @@
 namespace impactx::diagnostics
 {
 
-    /** Returns the three independent kinetic invariants
+    /** Returns the three independent kinematic moment invariants
      *  denoted I2, I4, and I6 as constructed from the 6x6
      *  beam covariance matrix.  These three quantities are invariant
      *  under any linear symplectic transport map, and are used in the
-     *  calculation of the three eigenemittances.
+     *  calculation of the three eigenemittances, as described in:
+     *
+     *  G. Rangarajan, F. Neri, and A. Dragt, "Generalized Emittance
+     *  Invariants," in Proc. 1989 IEEE Part. Accel. Conf., Chicago, IL,
+     *  1989, doi:10.1109/PAC.1989.73422.
+     *  A. Dragt, F. Neri, and G. Rangarajan, "General Moment Invariants
+     *  for Linear Hamiltonian Systems," Phys. Rev. A 45, 2572-2585 (1992),
+     *  doi:10.1103/PhysRevA.45.2572.
      *
      * @param[in] Sigma symmetric 6x6 covariance matrix
      * @returns tuple containing invariants I2, I4, and I6
@@ -43,7 +50,14 @@ namespace impactx::diagnostics
      *  beam covariance matrix.  These three quantities are invariant
      *  under any linear symplectic transport map, and reduce to
      *  the projected normalized rms emittances in the limit of
-     *  uncoupled transport.
+     *  uncoupled transport.  These quantities are described in:
+     *
+     *  G. Rangarajan, F. Neri, and A. Dragt, "Generalized Emittance
+     *  Invariants," in Proc. 1989 IEEE Part. Accel. Conf., Chicago, IL,
+     *  1989, doi:10.1109/PAC.1989.73422.
+     *  A. Dragt, F. Neri, and G. Rangarajan, "General Moment Invariants
+     *  for Linear Hamiltonian Systems," Phys. Rev. A 45, 2572-2585 (1992),
+     *  doi:10.1103/PhysRevA.45.2572.
      *
      * @param[in] Sigma symmetric 6x6 covariance matrix
      * @returns tuple containing eigenemittances e1, e2, and e3