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The Pion Mass Formula

The often used Gell-Mann-Oakes-Renner mass formula for Nambu-Goldstone (NG) bosons in QCD, such as the pions, involves the condensate $<\overline{q}q>$, $f_π$ and the quark current masses. Within the context of the Global Colour Model (GCM) for QCD a manifestly different formula was recently found by Cahill and Gunner. Remarkably Langfeld and Kettner have shown the two formula to be equivalent. Here we note that the above recent analyses refer to the constituent pion and not the exact pion, even within the GCM. Further we generalise the Langfeld-Kettner identity to include the full response of the constituent quark propagators to the presence of a non-zero (and running) quark current mass.