Before revision:  We prove that if the marginal production costs of both competing firms are equal then the equilibrium point is asymptotically stable and independent of time delay . As consequence, our model can not have bifurcations if the delay, as a parameter, is varied. It further imply that less tax evasion and higher public revenue can be achieved either by increasing the effectiveness of audits or by adjusting the penalties for tax evasion .
After revision:  We prove that our conditions actually imply asymptotically stable equilibrium and delay independence. Conditions include the same marginal cost and equal probability for evading taxes. We give examples of cost and inverse demand functions satisfying the proposed conditions. Some economic interpretations of our results are also included .