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worked, thank you so much!!!
If the region is becoming irregular, like the one, in the picture attached, is there a way in Mathematica to create irregular shapes by composing, subtracting images?![enter image description here][1] [1]:...
Ok. So the error message was helpful in finding the error in the pde's. Concerning the numerical problems I learned in former times, that sometimes a rescaling of the variables (concentration, time and lengths), so that constants have about the same...
Bill Simpson, thank you very much for both, your time and your help.