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n positive integer and d_{k}, k<=n its divisors

Source: bmo 1989

April 23, 2007
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Problem Statement

Let nn be a positive integer and let d1,d2,,,dkd_{1},d_{2},,\ldots ,d_{k} be its divisors, such that 1=d1<d2<<dk=n1=d_{1}<d_{2}<\ldots <d_{k}=n. Find all values of nn for which k4k\geq 4 and n=d12+d22+d32+d42n=d_{1}^{2}+d_{2}^{2}+d_{3}^{2}+d_{4}^{2}.