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could any one tell me how to find the sum of all positive integers less than $n$ and relatively prime to $n$? $n>1$

For $n=7$, I have $\phi(n)=6$ and the sum is ${6(6+1)\over 2}={n\over 2}.\phi(n)$, is that true for any $n$?

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  • $\begingroup$ This would be true for any prime $n$ (like your $7$), since all numbers less than $n$ would in that case be relatively prime. For any other number, not so much. $\endgroup$
    – Arthur
    Jul 31 '13 at 10:28
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It’s true for all $n>2$. The reason is that if $k\in\{1,\ldots,n-1\}$ is relatively prime to $n$, so is $n-k$, so the integers that you’re adding can be combined into $\frac{\varphi(n)}2$ pairs whose members sum to $n$. If $n>2$, $\frac{n}2$ is never relatively prime to $n$, so you really do get pairs $\{k,n-k\}$.

See OEIS A023896 for some references.

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