# I need help

Given a set $$\{1;2;3;4;5\}$$, find the number of 5 digits numbers that none of its consecutive digits are equal to each other,. If possible, find the sum of them

Note by Gurīdo Cuong
1 year, 8 months ago

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## Comments

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Assuming that the given set is the set of digits we can use (with replacement) to form the 5-digit numbers, the number of such 5-digit numbers with no two consecutive digits equal to each other is equal to $$5\times 4^4=1280$$. The combinatorial argument here is constructive: We can select the first digit in 5 ways, then the second digit in 4 ways (excluding the first digit), the third digit in 4 ways (excluding the second digit) and so on till the 5th digit. By the rule of product, there are $$1280$$ such numbers.

Now, to find the sum of all such numbers, note that every digit appears exactly $$4^4=256$$ times at each digit-place among all such numbers. This can be proved easily using the argument used above. If we fix a digit at a digit-place, its adjacent digits can be chosen in $$4$$ ways and their adjacent digits can also be chosen in $$4$$ ways since we can't have adjacent digits equal to each other. Repeating this argument shows that for each fixed digit at a fixed digit place, there are exactly $$256$$ numbers having the said digit at the said digit-place.

Hence, by the rule of product, the sum is given by, $256(1+2+3+4+5)(10^4+10^3+10^2+10+1)=42666240$

To generalize this to some extent, if $$S$$ be the set of digits that can be used to make $$n$$-digit numbers with no two consecutive digits equal to each other, then,

• If $$0\notin S$$, there are $$|S|\times (|S|-1)^{n-1}$$ such numbers and their sum is given by, $(|S|-1)^{n-1}\left(\sum_{k\in S}k\right)\left(\sum\limits_{k=0}^{n-1}10^k\right)=\frac{(|S|-1)^{n-1}(10^n-1)\left(\sum\limits_{k\in S}k\right)}9\\~\\$

• If $$0\in S$$, then there are $$(|S|-1)^n$$ such numbers. I'm pretty sure that there's a closed form for their sum, but it gets a bit complicated for me. I'm still working on it. I'll edit this comment when I get the closed form.

- 1 year, 8 months ago

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