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fix: include number/2 divisor in FriendlyNumbers sumDivisors #1904

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fix: include number/2 divisor in FriendlyNumbers sumDivisors #1904

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fix: include number/2 divisor in FriendlyNumbers sumDivisors 
sumDivisors looped 'for (let i = 0; i < number / 2; i++)', which excludes
i === number/2 -- a real divisor of every even number -- so the divisor
sum (and abundancy index) was wrong for even inputs, e.g. sumDivisors(6)
returned 9 instead of 12 and sumDivisors(28) returned 42 instead of 56.
That made FriendlyNumbers misclassify pairs (e.g. FriendlyNumbers(15, 20)
returned true though their abundancy indices differ).

Loop from 1 to number/2 inclusive. Added a test file (the module had none).
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Shaked Shlomo Shaked Shlomo
Shaked Shlomo authored and Shaked Shlomo committed Jun 1, 2026
commit 3dd22dd54c66414517e211207fb640e12d177a4f
2 changes: 1 addition & 1 deletion Maths/FriendlyNumbers.js
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Original file line number Diff line number Diff line change
Expand Up @@ -29,7 +29,7 @@ function abundancyIndex(number) {

function sumDivisors(number) {
let runningSumDivisors = number
for (let i = 0; i < number / 2; i++) {
for (let i = 1; i <= number / 2; i++) {
if (Number.isInteger(number / i)) {
runningSumDivisors += i
}
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21 changes: 21 additions & 0 deletions Maths/test/FriendlyNumbers.test.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
import { FriendlyNumbers } from '../FriendlyNumbers'

describe('FriendlyNumbers', () => {
it('should return true for the perfect-number pair 6 and 28', () => {
expect(FriendlyNumbers(6, 28)).toBe(true)
})

it('should return true for the friendly pair 30 and 140', () => {
expect(FriendlyNumbers(30, 140)).toBe(true)
})

it('should return false for numbers with different abundancy', () => {
expect(FriendlyNumbers(15, 20)).toBe(false)
})

it('should throw for invalid input', () => {
expect(() => FriendlyNumbers(6, 6)).toThrow()
expect(() => FriendlyNumbers(0, 5)).toThrow()
expect(() => FriendlyNumbers(1.5, 5)).toThrow()
})
})
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