# Euler Problem 12: Highly Divisible Triangular Number

The divisors of 10 illustrated with Cuisenaire rods: 1, 2, 5, and 10 (Wikipedia).

Euler Problem 12 takes us to the realm of triangular numbers and proper divisors.

The image on the left shows a hands-on method to visualise the number of divisors of an integer. Cuisenaire rods are learning aids that provide a hands-on way to explore mathematics.

## Euler Problem 12 Definition

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: $1, 3, 6, 10, 15, 21, 28, 36, 45, 55, \ldots$ 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?

## Solution

Vishal Kataria explains a simple method to determine the number of divisors using prime factorization as explained by in his video below. The prime factorization of $n$ is given by:

$n = p^{\alpha_1}_1 \times p^{\alpha_2}_2 \times p^{\alpha_k}_k$

The number of proper divisors is:

$d = (\alpha_1 + 1) (\alpha_2 + 1) \ldots (\alpha_k + 1)$

The code reuses the prime factorisation function developed for Euler Problem 3. This function results in a vector of all prime factors, e.g. the prime factors of 28 are 2, 2 and 7.

The code to solve this problem determines the values for alpha using the run length function. This function counts the number of times each element in a sequence is repeated. The outcome of this function is a vector of the values and the number of times each is repeated. The prime factors of 28 are 2 and 7 and their run lengths are 2 and 1. The number of divisors can now be determined.

$28 = 2^2 \times 7^1$

$d = (2+1)(1+1) = 6$

The code to solve Euler Problem 12 is shown below. The loop continues until it finds a triangular number with 500 divisors. The first two lines increment the index and create the next triangular number. The third line in the loop determines the number of times each factor is repeated (the run lengths). The last line calculates the number of divisors using the above-mentioned formula.

i <- 0
divisors <- 0
while (divisors < 500) {
i <- i + 1
triangle <- (i * (i+1)) / 2
pf <- prime.factors(triangle)
alpha <- rle(pf)
divisors <- prod(alpha\$lengths+1)
}


# Euler Problem 5: Smallest Multiple

Euler Problem 5 relates to the divisibility of numbers.

## Euler Problem 5

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 by all of the numbers from 1 to 20?

## Solution

The solution will also be divisible by the number 1 to 10 so we can start at 2520 and increment by 2520. The loop checks whether the number is divisible by the numbers 1 to 20.

# Start as high as possible
i <- 2520
# Check consecutive numbers for divisibility by 1:20
while (sum(i%%(1:20)) != 0) {
i <- i + 2520 # Increase by smallest number divisible by 1:10
}


# Euler Problem 1: Multiples of 3 or 5

## Euler Problem 1 Definition

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below 1000.

## Solution

There are four ways to solve this problem in R.

1. Brute force loop through all numbers from 1 to 999 and test whether they are divisible by 3 or by 5 using the modulus function.
2. Vector arithmetics.
3. Sequences of 3 and 5, excluding duplicates (numbers divisible by 15).
4. Using an arithmetic approach.

By the way, the problem definition on the Project Euler website is not consistent: the title mentions multiples of 3 AND 5, while the description asks for multiples of 3 OR 5.

# Solution 1
for (i in 1:999) {
if (i%%3 == 0 | i%%5 == 0)
}

# Solution 2
sum((1:999)[((1:999)%%3 == 0) | ((1:999)%%5 == 0)])

# Solution 3
sum(unique(c(seq(3, 999, 3), seq(5, 999, 5))))


These three brute-force solutions all take time to process, especially when using targets much igher than 999. An analytical solution will significantly reduce the processing time.

## Analytical Solution

The sum of an arithmetic progression , where n is the number of elements and a1 and an are the lowest and highest value, is:

$\mathrm{sum}= \frac{n(a_{1} + a_n)}{2}$

The numbers divisible by $n=3$ can be expressed as:

$\mathrm{sum}_3(999)=3+6+9+12+ \ldots +999 = 3(1+2+3+4+ \ldots +333)$

We can now calculate the sum of all divisors by combining the above progression with the formula for arithmetic progressions as expressed in the above code, where m is the divisor and n the extent of the sequence.

p is the highest number less than n divisible by m. In the case of 5, this number is 995.

$p = n \lfloor (m/n) \rfloor$

Substitution gives:

$\mathrm{sum}_m(n) = p \frac{1+(p/m)}{2}$

# Solution 4
SumDivBy <- function(m, n) {
p <- floor(n/m)*m # Round to multiple of n
return (p*(1+(p/m))/2)
}

answer <- SumDivBy(3, 999) + SumDivBy(5, 999) - SumDivBy(15, 999)