Maths Olympiad Prep

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Problem 610

Number theory Difficulty 2.7 Find the answer CEMC Fermat

What is the sum of the positive divisors of 1184?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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Official solution

We start by finding the prime factors of 1184: 1184=2⋅592=22⋅296=23⋅148=24⋅74=25⋅371184=2 \cdot 592=2^{2} \cdot 296=2^{3} \cdot 148=2^{4} \cdot 74=2^{5} \cdot 37. The positive divisors of 1184 cannot contain prime factors other than 2 and 37, and cannot contain more than 5 factors of 2 or 1 factor of 37. Thus, the positive divisors are 1,2,4,8,16,32,37,74,148,296,592,11841,2,4,8,16,32,37,74,148,296,592,1184. The sum, SS, of these divisors is S=1+2+4+8+16+32+37+74+148+296+592+1184=(1+2+4+8+16+32)+37⋅(1+2+4+8+16+32)=(1+2+4+8+16+32)⋅(1+37)=63⋅38=2394S = 1+2+4+8+16+32+37+74+148+296+592+1184 = (1+2+4+8+16+32)+37 \cdot(1+2+4+8+16+32) = (1+2+4+8+16+32) \cdot(1+37) = 63 \cdot 38 = 2394.

Source: Omni-MATH, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.