We have to form a number of n digits having digits from 1 to 9. Constraint is that first and last digit must be same and no two consecutive digits must be same. How many such number of n digits can be there?
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$n$ has to be larger than $2$ then? Try some examples...! Is this homework? – draks ... Oct 05 '12 at 06:48
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Not a homework. Came through it while practicing a problem. – Shashwat Kumar Oct 05 '12 at 06:52
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I guess $n=1$ would fit the constraints given here. – Oct 05 '12 at 06:57
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Since the first and last digits have to be the same, this is the same as asking how many ways there are to colour $n-1$ points on a circle with $9$ colours such that any two adjacent points have different colours. The answer is given here: $(-1)^{n-1}(9-1)+(9-1)^{n-1}=(-1)^{n-1}\cdot8+8^{n-1}$
