Sandbox::https://github.com/sibalonecoding-code/JavaScript/blob/master/Maths/FriendlyNumbers.js

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'In number theory, friendly numbers are two or more natural numbers with a common abundancy index, the

ratio between the sum of divisors of a number and the number itself.'

Source: https://en.wikipedia.org/wiki/Friendly_number

See also: https://mathworld.wolfram.com/FriendlyNumber.html#:~:text=The%20numbers%20known%20to%20be,numbers%20have%20a%20positive%20density.

export const FriendlyNumbers = (firstNumber, secondNumber) => {

// input: two integers

// output: true if the two integers are friendly numbers, false if they are not friendly numbers

// First, check that the parameters are valid

if (

!Number.isInteger(firstNumber) ||

!Number.isInteger(secondNumber) ||

firstNumber === 0 ||

secondNumber === 0 ||

firstNumber === secondNumber

) {

throw new Error('The two parameters must be distinct, non-null integers')

}

return abundancyIndex(firstNumber) === abundancyIndex(secondNumber)

}

function abundancyIndex(number) {

return sumDivisors(number) / number

}

function sumDivisors(number) {

let runningSumDivisors = number

for (let i = 0; i < number / 2; i++) {

if (Number.isInteger(number / i)) {

runningSumDivisors += i

}

return runningSumDivisors

}