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I would like to show that for any positive integers $d_1, \dots, d_r$ one has $$ \sum_{i=1}^r (-1)^{i+1}\biggl( \sum_{1\leq k_1 < \dots < k_i \leq r} \gcd(d_{k_1}, \dots , d_{k_i})\biggr) ~\leq~ \prod_{i=1}^r\biggl( \prod_{1\leq k_1 < \dots < k_i \leq r} \gcd(d_{k_1}, \dots , d_{k_i}) \biggl)^{(-1)^{i+1}}. $$ Note that the rhs of the upper inequality is exactly $\operatorname{lcm}(d_1,\dots,d_r)$. Also note that if we denote the lhs of the upper equation by $L(d_1, \dots, d_r)$, then one has that $$ L(d_1, \dots, d_r) = L(d_1, \dots, d_{r-2}, d_{r-1}) + L(d_1, \dots, d_{r-2}, d_{r}) - L(d_1, \dots, d_{r-2}, \text{gcd}(d_{r-1},d_r)). $$

Thanks for the help!

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    I compute {1,1,2} as a counter-example. It looks like you want strict inequality $k_1. In this case, however, {1,2} is a counter-example to the claim "equality only if $d_1=\cdots=d_r$" (but the inequality doesn't seem to have a small counter-example).2012-09-25

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