Common Mistakes


      a^N b^N c^N is an element of L => L is not CF
bullet Trying to prove L is not CF
bullet L includes strings of the form a^N b^N c^N
bullet This is NOT enough to say that L is not context free
bullet Both a* b* c* and a^N b^N c^N include strings of the form a^N b^N c^N, the first is regular and context free, the second is not context free.

      Using pumping to show resulting string is not in w
bullet L = a^n b^n*m c^n. Trying to show L is not CF.
bullet You can pick the value of m for a particular string, such as m=1, w = a^2M b^2M c^2M
bullet You CANNOT pick the value of m for the language, the language is given to you, you cannot change it. It accepts strings for all values of m.
bullet Use pumping on w to show the resulting string is not in L.