# In simple terms what is the sequence 220,171700,167167000,166716670000 a part of?

I know that

<1,2,3,...,10>$\cdot$<1 0,9,8,...1>=220

<1,2,3,...,100>$\cdot$<100,99,98,...,1>=171700

<1,2,3,...,1000>$\cdot$<1000,999,998,...,1>=167167000

<1,2,3,...,10000>$\cdot$<10000,9999,9998,...,1>=166716670000 

And 2,17,167,1667 is a part of the OEIS A126109 (5*10^n+1)/3. But in simple terms what is the sequence 220,171700,167167000,166716670000` a part of?

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What do you mean by "a part of"? Are you looking for a simple expression for the $n$th term? –  Matthew Conroy Jan 7 '14 at 21:41

## 2 Answers

The $n$th term of your sequence is the $10^{n}$th term of A000292, that is your $n$th term is $$\frac{10^n(10^n+1)(10^n+2)}{6}$$

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We have $$<1,2,3,...,n> \cdot <n,n-1,n-2,...,1>$$ $$= \sum_{i=1}^n (n+1-i)i = \sum_{i=1}^n i(n+1) - \sum_{i=1}^n i^2$$ $$= (n+1) \frac{1}{2}n(n+1) - \frac{1}{6}n(n+1)(2n+1) = \frac{1}{6}n(n+1)(n+2).$$

Replacing $n$ by $10^n$ yields the general term of your sequence.

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+1 for figuring out what the OP's notation means. –  Ilmari Karonen Jan 8 '14 at 2:02