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n-digit numbers created using digits 5 and 9, where no four identical digits stand side by side.

Posted on 2015-02-12
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How many n-digit numbers can be created using only digits 5 and 9, where no four identical digits stand side by side?
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Question by:Nusrat Nuriyev
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ozo earned 500 total points
ID: 40606194
a[1] = 2
a[2] = 4
a[3] = 8
a[n] = a[n - 1] + a[n - 2] + a[n - 3]
https://oeis.org/A135491
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by:Nusrat Nuriyev
ID: 40606261
ozo, also, this a[n]=a[n-1]+a[n-2]+a[n-3] gives us the n-digit numbers which can be created using only digits 5 and 9, where no three ones stand side by side. (but this does not mentioned in OEIS)

Could you explain why this sequence solves my original question? For better understanding.
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