dirac_trace() can handle arbitrary gamma strings without gamma5. The only

remaining unimplemented case is now Tr[gamma5 S_2n]

diff --git a/ginac/clifford.cpp b/ginac/clifford.cpp
index eb1629d4957aad08f5a001ac2348dc89752a07bb..d346c44ad3fd4548226c7f147908dd5fa3b11016 100644 (file)
--- a/ginac/clifford.cpp
+++ b/ginac/clifford.cpp
@@ -363,6 +363,28 @@ ex dirac_trace(const ex & e, unsigned char rl = 0)
                        // Tr gamma_mu gamma_nu = 4 g_mu_nu
                        if (num == 2)
                                return 4 * lorentz_g(e.op(0).op(1), e.op(1).op(1));
+
+                       // Traces of 4 or more gammas are computed recursively:
+                       // Tr gamma_mu1 gamma_mu2 ... gamma_mun =
+                       //   + eta_mu1_mu2 * Tr gamma_mu3 ... gamma_mun
+                       //   - eta_mu1_mu3 * Tr gamma_mu2 gamma_mu4 ... gamma_mun
+                       //   + eta_mu1_mu4 * Tr gamma_mu3 gamma_mu3 gamma_mu5 ... gamma_mun
+                       //   - ...
+                       //   + eta_mu1_mun * Tr gamma_mu2 ... gamma_mu(n-1)
+                       exvector v(num - 2);
+                       int sign = 1;
+                       const ex &ix1 = e.op(0).op(1);
+                       ex result;
+                       for (int i=1; i<num; i++) {
+                               for (int n=1, j=0; n<num; n++) {
+                                       if (n == i)
+                                               continue;
+                                       v[j++] = e.op(n);
+                               }
+                               result += sign * lorentz_g(ix1, e.op(i).op(1)) * dirac_trace(ncmul(v), rl);
+                               sign = -sign;
+                       }
+                       return result;
                }
 
                throw (std::logic_error("dirac_trace: don't know how to compute trace"));

diff --git a/ginac/clifford.h b/ginac/clifford.h
index 4d771530386b842320ba846853a81fa5a005212a..f1783396995620d63c1a38f24315675f97476742 100644 (file)
--- a/ginac/clifford.h
+++ b/ginac/clifford.h
@@ -126,7 +126,10 @@ ex dirac_gamma(const ex & mu, unsigned char rl = 0);
 ex dirac_gamma5(unsigned char rl = 0);
 
 /** Calculate the trace of an expression containing gamma objects with
- *  a specified representation label.
+ *  a specified representation label. The computed trace is a linear
+ *  functional that is equal to the usual trace only in D = 4 dimensions.
+ *  In particular, the functional is non-cyclic in D != 4 dimensions when
+ *  gamma5 is involved.
  *
  *  @param rl Representation label */
 ex dirac_trace(const ex & e, unsigned char rl = 0);