diff --git a/notebooks/problem0021.ipynb b/notebooks/problem0021.ipynb
index 64b2ef0..d0469c9 100644
--- a/notebooks/problem0021.ipynb
+++ b/notebooks/problem0021.ipynb
@@ -122,7 +122,7 @@
     "Therefore, if you have the number's factorization (see [problem 3](https://projecteuler.net/problem=3)), you can use it to compute the sum of its divisors.\n",
     "\n",
     "## Avoiding factoring\n",
-    "Since we need all the sums of divisors up to 10000, instead of factoring each number individually, we can compute each value of $\\sigma$ in a loop, once again taking advantage of $\\sigma$ being multiplicative. (In some ways, this method is similar to the sieve of Eratosthenes - see [problem 10](https://projecteuler.net/problem=10))"
+    "Since we need all the sums of divisors up to 10000, instead of factoring each number individually, we can compute each value of $\\sigma$ in a loop, once again taking advantage of $\\sigma$ being multiplicative. (In some ways, this method is similar to the sieve of Eratosthenes - see [problem 10](https://projecteuler.net/problem=10).)"
    ]
   },
   {