Positively Described Numbers - Part 2

Surely you've heard of the perfect numbers, numbers that have the property that the sum of the proper factors of the number are equal to the number. The first few examples of perfect numbers are 6 , 6, 28 , 28, and 496. 496. Because the sum of the proper factors of perfect numbers is equal to the number, the sum of all \textit{all} of the factors of the number is equal to twice the number. Because of this, perfect numbers are called 2 -perfect. 2\text{-perfect.}

A number is k -perfect k\text{-perfect} if the sum of the factors of the number is equal to k k times the number. Let { a 1 , a 2 , a 3 , a n , } \{a_1,a_2,a_3\ldots,a_n,\ldots\} be the set of numbers that are the smallest k -perfect k\text{-perfect} number for increasing values of k , k, starting at 1. 1. a n a_n is the smallest n -perfect n\text{-perfect} number. What is the sum of the digits of this sum? i = 1 5 a i \sum_{i=1}^5a_i ................................................................................................................... \text{...................................................................................................................} Hint: \textbf{Hint:} It will take a really long time for you to solve this directly. How can you optimize the amount of numbers you need to check?


The answer is 46.

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2 solutions

Ronald Overwater
Mar 18, 2014

http://epress.lib.uts.edu.au/research/bitstream/handle/10453/20034/02Whole.pdf?sequence=2021

Getting the first four numbers was easy, but for the fifth one I used the procedure of working with primes only....

Ronald Overwater - 7 years, 2 months ago

I used a 20-line long code to find 4-perfect but the fifth one had its numbers well beyond what I was expecting it to be. I must admit that I did a little internet search to see if someone had done some analysis on the patter of this k-perfect number. I really feel bad that I accidentally got to know what 5-perfect was. According to the paper to which Ronald posted its link, there have been extensive studying on this concept (the group of researchers include Descartes and Mersenne) and in 2002 the smallest and yet a single 11-perfect was discovered. (This information may be outdated.) What an interesting concept people delved into.

Simon Seo - 7 years, 2 months ago
Finn Hulse
Apr 7, 2014

Read this .

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