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Thank you very much! Although I can't understand your words completely(Because of my poor mathematical level).

I have a question when reading Concrete Mathematics.It's in Unit 2(SUMS) section 7(INFINITE SUMS).Authors have proved that ∑k>=0xk=1/(1-x) (0<=x<1), but they also say: We might also try setting x=-1 in the formula ∑k>=0xk=1/(1-x), since we've proved that this formula holds when 0<=x<1.(Maybe It's on Page 59) I don't know why they can set x=-1 and use the formula correctly. The formula is right when 0<=x<1, isn't it? (PS: My English is really poor. Please point out my mistakes. Thank you!)