Suppose there are N positions.(adsbygoogle = window.adsbygoogle || []).push({});

For each position, one can fill it with S,F or T.

There is one constraint that F and T cannot be next to each other. This means that a filling with FT in the sequence or TF in the sequence is not allowed.

For example, if N = 5. We have FSSTT, SFSTT are valid sequences, but SFTFS is not.

Can anyone help me with calculating the number of possible sequences?

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# A combinatorial problem

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