2002 IMO Problems/Problem 4

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Problem: Let $n>1$ be an integer and let $1=d_{1}<d_{2}<d_{3} \cdots <d_{r}=n$ be all of its positive divisors in increasing order. Show that \[d=d_1d_2+d_2d_3+ \cdots +d_{r-1}d_r <n^2\]

Solution 1