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Dear Mr. Miller, Sorry for the late reply but thank you! The problem did re-occur but after your suggestion holding down the SHIFT key Mathematica is working fine.
Is there a possibility in Notebook to give in a part of a series and solve this for the n-th number. E.g. {1, 2, 5, 12, 29, 70, 169, 408} these are the first 8 Pell-numbers. How do I calculate the 9th number? I know that the formula is ((1 +...