FriendlyNumbers.js

/*
 '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
}