diff --git a/src/hackerrank/interview_preparation_kit/dynamic_programming/max_array_sum.test.js b/src/hackerrank/interview_preparation_kit/dynamic_programming/max_array_sum.test.js
index 34ae268d..409b8583 100644
--- a/src/hackerrank/interview_preparation_kit/dynamic_programming/max_array_sum.test.js
+++ b/src/hackerrank/interview_preparation_kit/dynamic_programming/max_array_sum.test.js
@@ -22,7 +22,7 @@ describe('max_array_sum', () => {
       expect(answer).toStrictEqual(test.expected);
     });
 
-    expect(TEST_CASES).toHaveLength(3);
+    expect(ALL_TEST_CASES).toHaveLength(4);
   });
 
   it('maxSubsetSum edge case zero', () => {
diff --git a/src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase.test.js b/src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase.test.js
index 2775b7c3..fc6fa335 100644
--- a/src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase.test.js
+++ b/src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase.test.js
@@ -41,6 +41,6 @@ describe('ctci_recursive_staircase', () => {
       });
     });
 
-    expect(TEST_CASES).toHaveLength(3);
+    expect(TEST_CASES_GENERALIZED).toHaveLength(3);
   });
 });
diff --git a/src/projecteuler/helpers/divisors.js b/src/projecteuler/helpers/divisors.js
index 8feede2d..f239d2dc 100644
--- a/src/projecteuler/helpers/divisors.js
+++ b/src/projecteuler/helpers/divisors.js
@@ -112,6 +112,9 @@ export const primeFactors = (target) => {
   return { 'factors': factors, 'cycles': cycles };
 };
 
+export const isPrime = (target) =>
+  target !== 1 && target === nextPrimeFactor(target).factor;
+
 export const abundance = (target) => {
   const theDivisors = properDivisors(target);
   const divSum = sum(theDivisors);
diff --git a/src/projecteuler/helpers/divisors.test.js b/src/projecteuler/helpers/divisors.test.js
index cb1d8b47..f91cbc01 100644
--- a/src/projecteuler/helpers/divisors.test.js
+++ b/src/projecteuler/helpers/divisors.test.js
@@ -1,15 +1,17 @@
 import { describe, expect, it } from '@jest/globals';
 import {
-  divisors,
   abundance,
+  divisors,
+  isPrime,
   nextPrimeFactor,
   primeFactors,
+  properDivisors,
   ___DIVISORS_DEFICIENT___,
   ___DIVISORS_PERFECT___,
   ___DIVISORS_ABUNDANT___
 } from './divisors.js';
 
-describe('divisors of a number', () => {
+describe('divisors and prime numbers', () => {
   it('divisors of one', () => {
     expect.assertions(1);
 
@@ -35,6 +37,16 @@ describe('divisors of a number', () => {
     ]);
   });
 
+  it('proper divisors of a number', () => {
+    expect.assertions(5);
+
+    expect(properDivisors(1)).toStrictEqual([]);
+    expect(properDivisors(2)).toStrictEqual([1]);
+    expect(properDivisors(8)).toStrictEqual([1, 2, 4]);
+    expect(properDivisors(9)).toStrictEqual([1, 3]);
+    expect(properDivisors(16)).toStrictEqual([1, 2, 4, 8]);
+  });
+
   it('next prime factor of a target number', () => {
     expect.assertions(5);
 
@@ -81,6 +93,24 @@ describe('divisors of a number', () => {
     });
   });
 
+  it('some numbers are prime', () => {
+    expect.assertions(4);
+
+    expect(isPrime(1)).toBe(false);
+    expect(isPrime(2)).toBe(true);
+    expect(isPrime(7)).toBe(true);
+    expect(isPrime(13)).toBe(true);
+  });
+
+  it('some numbers are not prime', () => {
+    expect.assertions(4);
+
+    expect(isPrime(4)).toBe(false);
+    expect(isPrime(10)).toBe(false);
+    expect(isPrime(100)).toBe(false);
+    expect(isPrime(3000)).toBe(false);
+  });
+
   it('abundance of a integer number', () => {
     expect.assertions(3);