Added a new Maths algorithm to determine if two non-null integers are "friendly numbers" (#1267)

Author: fun-guava

Maths/FriendlyNumbers.js

@@ -0,0 +1,32 @@
+/*
+  '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
+}