A unitary divisor of a number n is a number d such that d|n and gcd(d, n/d)=1. For example, 3 is a unitary divisor of 12 because gcd(3, 12/3)=gcd(3, 4)=1.

A unitary perfect number is a number that is the sum of its unitary divisors less than itself. For example, 60 is a unitary perfect number because its unitary divisors less than itself are 1, 3, 4, 5, 12, 15, and 20, and 1+3+4+5+12+15+20=60. Find all unitary perfect numbers less than 1000000.

(In reply to re(2): just look up by SH by Ady TZIDON)

I think the essence of the current discussion is best exemplified by the fact that, just to take myself: I like to do KenKen puzzles. I don't use computer code to solve them, though I did write a program to solve them, as using that program over and over again would be silly. But writing it once was a challenge.

But back to the point: books of KenKen puzzles have the answers in the back of the book. But it's silly to fill in the blank puzzle sheed with the answers from the back of the book.  Yes, the answers are known by someone, but the essence of the puzzle is to find it oneself.

Posted by Charlie on 2013-12-17 10:04:18