## Problem 699, error in provided example?

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sOvr9000
Posts: 1
Joined: Sun Feb 02, 2020 7:27 pm

### Problem 699, error in provided example?

It is possible that I'm wrong, but I think there's an error in an example given in Problem 699. The example in question is T(100)=270.
The sigma function example is good, and a naive implementation of summing the factors of a positive integer will show that σ(10)=1+2+5+10=18.
The naive implementation to which I am referring is one where the square root (if it is an integer) of n is added twice. This incorrect implementation allows T(100)=270, but correcting the function to only count the square root once (if the square root is an integer) makes T(100)=258.
I specifically tested my factorization of 100, and when I was getting T(100)=270, I was counting the factor 10 twice. When I changed sigma to only count the square root once for general n, I got T(100)=258.
Am I just missing an important detail, or is my assumption correct that the square root should only be counted once?

EDIT: I also tested T(106). I got 26089275 instead of 26089287 using what I think is the correct sigma function, based on the definition given in the problem.

hk
Posts: 10660
Joined: Sun Mar 26, 2006 9:34 am
Location: Haren, Netherlands

### Re: Problem 699, error in provided example?

No, the given example is correct.
The triffle numbers not exceeding 100 are:

Code: Select all

n	sigma	sigma(n)/n
3	4      4/3
9  	13	13/9
12 	28	7/3
27	40	40/27
54	120	20/9
81	121	121/81
84	224	8/3

The answer for 10^6 is also correct.
There must be another error in your program.
(it is correct that the divisor 10 of 100 should only be added once.)