# Prime factorisation

## Project Euler 12 – Revisited

A while ago I treated Problem 12 of Project Euler and came up with several solutions as seen here.

Lets just repeat the problem here

The sequence of triangle numbers is generated by adding the natural numbers. So the 7th triangle number would be 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28. The first ten terms would be:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, …

Let us list the factors of the first seven triangle numbers:

1: 1
3: 1,3
6: 1,2,3,6
10: 1,2,5,10
15: 1,3,5,15
21: 1,3,7,21
28: 1,2,4,7,14,28We can see that 28 is the first triangle number to have over five divisors.

What is the value of the first triangle number to have over five hundred divisors?

Posted by Kristian in Project Euler, 4 comments

## Project Euler 23: Find the sum of all the positive integers which cannot be written as the sum of two abundant numbers.

Problem 23 of Project Euler is about factorisation of a number once again, and has a rather long description. So let me give that to you right away

A perfect number is a number for which the sum of its proper divisors is exactly equal to the number. For example, the sum of the proper divisors of 28 would be 1 + 2 + 4 + 7 + 14 = 28, which means that 28 is a perfect number.

A number n is called deficient if the sum of its proper divisors is less than n and it is called abundant if this sum exceeds n.

As 12 is the smallest abundant number, 1 + 2 + 3 + 4 + 6 = 16, the smallest number that can be written as the sum of two abundant numbers is 24. By mathematical analysis, it can be shown that all integers greater than 28123 can be written as the sum of two abundant numbers. However, this upper limit cannot be reduced any further by analysis even though it is known that the greatest number that cannot be expressed as the sum of two abundant numbers is less than this limit.

Find the sum of all the positive integers which cannot be written as the sum of two abundant numbers.

I have only found one effective approach for this problem in C#, there might be more but I haven’t found them. My approach can be summaries in three steps

1. Find all abundant numbers
2. Create and mark all number which can be created as the sum of two abundant numbers
3. Sum up all non-marked numbers
Posted by Kristian in Project Euler, 23 comments

## Project Euler 21: Sum of Amicable Pairs Under 10000

After a few exercises with the focus on other areas, we are in Problem 21 of Project Euler back to focusing on number theory and factorisation. The problem reads

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
If d(a) = b and d(b) = a, where ab, then a and b are an amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.

Posted by Kristian in Project Euler, 14 comments

## Project Euler 12: Triangle Number with 500 Divisors

Problem 12 of Project Euler has a wording which is somewhat different than previous problems. However, as we shall see deriving efficient solutions for the problem, we can use theory which is very similar to some of the previous problems.  The problem reads

The sequence of triangle numbers is generated by adding the natural numbers. So the 7th triangle number would be 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28. The first ten terms would be:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, …

Let us list the factors of the first seven triangle numbers:

1: 1
3: 1,3
6: 1,2,3,6
10: 1,2,5,10
15: 1,3,5,15
21: 1,3,7,21
28: 1,2,4,7,14,28
We can see that 28 is the first triangle number to have over five divisors.

What is the value of the first triangle number to have over five hundred divisors?

Posted by Kristian in Project Euler, 31 comments

## Project Euler – Problem 5

Lets jump right into solving Problem 5 of Project Euler and let me try to give you an answer on how to solve it. The problem formulation is :

2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.

What is the smallest positive number that is evenly divisible (divisible with no remainder) by all of the numbers from 1 to 20?

Once again I will provide you with two different solutions and some tips and tricks on how to speed them up a bit.

Posted by Kristian in Project Euler, 34 comments

## Project Euler – Problem 3

I am sorry, I haven’t posted anything for a while. I have been busy moving, and is currently without an Internet connection. However I couldn’t keep away any more.

Problem 3 in Project Euler reads:

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

I used two different approaches for this, and lets get right to them. Continue reading →

Posted by Kristian in Project Euler, 24 comments