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Representation of fraction as sum of unit fractions

Source: SAMO 2017 Q3

July 30, 2017
egyptian fractionsnumber theory

Problem Statement

A representation of 1720\frac{17}{20} as a sum of reciprocals 1720=1a1+1a2++1ak \frac{17}{20} = \frac{1}{a_1} + \frac{1}{a_2} + \cdots + \frac{1}{a_k} is called a calm representation with kk terms if the aia_i are distinct positive integers and at most one of them is not a power of two.
(a) Find the smallest value of kk for which 1720\frac{17}{20} has a calm representation with kk terms. (b) Prove that there are infinitely many calm representations of 1720\frac{17}{20}.