An engineers' dream come true
Problem 263Published on Saturday, 7th November 2009, 01:00 am; Solved by 721; Difficulty rating: 75%
Consider the number 6. The divisors of 6 are: 1,2,3 and 6.
Every number from 1 up to and including 6 can be written as a sum of distinct divisors of 6:
1=1, 2=2, 3=1+2, 4=1+3, 5=2+3, 6=6.
A number n is called a practical number if every number from 1 up to and including n can be expressed as a sum of distinct divisors of n.
A pair of consecutive prime numbers with a difference of six is called a sexy pair (since "sex" is the Latin word for "six"). The first sexy pair is (23, 29).
We may occasionally find a triple-pair, which means three consecutive sexy prime pairs, such that the second member of each pair is the first member of the next pair.
We shall call a number n such that :
- (n-9, n-3), (n-3,n+3), (n+3, n+9) form a triple-pair, and
- the numbers n-8, n-4, n, n+4 and n+8 are all practical,
Find the sum of the first four engineers’ paradises.