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It is well known that for two numbers $a $ and $b$, $$\text {lcm} (a,b)\times \text {hcf} (a,b)=ab$$ Does there exist a similar equality/ inequality between HCF, LCM and product of multiple numbers? (i.e. some relation between $\text {hcf} (a_1,a_2,...,a_n)$, $\text{lcm} (a_1,a_2,...,a_n)$ and $a_1\times a_2 \times ...\times a_n$)

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marked as duplicate by Ng Chung Tak, Jeremy Rickard, Henrik, Kushal Bhuyan, Davide Giraudo Jul 15 '16 at 10:59

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  • $\begingroup$ The product of the lcm and hcf of $(2,2,2)$ is $4$. For $(1,1,8)$ it is $8$. Same product. $\endgroup$ – André Nicolas Jul 15 '16 at 5:32
  • $\begingroup$ I asked for a similar equality/ inequality, not an exact one. $\endgroup$ – Apurv Jul 15 '16 at 5:34
  • $\begingroup$ In general there will be an inclusion-exclusion equality relating $lcm(a_1,\ldots,a_n)$ to the $gcd$ of each subset of $\{a_1,\ldots,a_n\}$. See en.wikipedia.org/wiki/Inclusion%E2%80%93exclusion_principle $\endgroup$ – stewbasic Jul 15 '16 at 5:36
  • $\begingroup$ @Apury: The fact that in the example the products $a\times b\times c$ are different shows that the product of lcm and hcf is not a function of the product of the three numbers. $\endgroup$ – André Nicolas Jul 15 '16 at 5:37
  • $\begingroup$ I know an inequalitu that relates the first n natural numbers. So i was thinkingthat could probably be done for any n numbers. Any suggestions? $\endgroup$ – Apurv Jul 15 '16 at 5:40
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You cannot express $a_1a_2\dots a_n$ in terms of $\gcd(a_1,a_2,\dots,a_n)$ and $\operatorname{lcm}(a_1,a_2,\dots,a_n)$ in any way if $n>2$. For instance, if $n=3$, note that $\{2,4,16\}$ and $\{2,8,16\}$ have the same GCD and LCM, but they have different products. You can find similar examples for any $n>2$.

For an inequality, I don't know what sort of thing you're looking for. But it is straightforward to see that $$\gcd(a_1,a_2,\dots,a_n)\operatorname{lcm}(a_1,a_2,\dots,a_n)\leq a_1a_2\dots a_n$$ whenever $n\geq 2$ (just compare the $p$-adic valuations of both sides for each prime $p$--the left-hand side is the sum of the valuations of two of the $a_i$, while the right-hand side is the sum of the valuations of all of them).

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  • $\begingroup$ Any inequality you are aware of? I know one that relates the first n natural numbers. $\endgroup$ – Apurv Jul 15 '16 at 5:38
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Not with just product.

The product of the lcm and hcf of $(2,2,2)$ is $4$. For $(1,1,8)$ it is $8$. But $2\times 2\times 2=1\times 1\times 8$.

Thus the product of lcm and hcf is not a function of the product of the three numbers.

Similarly, note that $(2,2,4)$ and $(2,4,4)$ have the same lcm and hcf. But their products are different, so the product of three numbers is not a function of their lcm and hcf.

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