diff --git a/extras/index.md b/extras/index.md
index 284736a..4576f4d 100644
--- a/extras/index.md
+++ b/extras/index.md
@@ -20,6 +20,6 @@ group theory, the following books and lecture notes are
 a recommended reading.
 
 * D. F. Holt, B. Eick, and E. A. O’Brien, *Handbook of computational group theory, Discrete Mathematics and its Applications* (Boca Raton), Chapman & Hall/CRC, Boca Raton, FL, 2005.
-* A. Hulpke, *Computational Group Theory* (2010), available at http://www.math.colostate.edu/ ~hulpke/CGT/cgtnotes.pdf.
+* A. Hulpke, *Computational Group Theory* (2010), available at [http://www.math.colostate.edu/~hulpke/CGT/cgtnotes.pdf](http://www.math.colostate.edu/~hulpke/CGT/cgtnotes.pdf).
 * Á. Seress, *Permutation group algorithms*, Cambridge Tracts in Mathematics, vol. 152, Cambridge University Press, Cambridge, 2003.
 * C. C. Sims, *Computation with finitely presented groups*, Encyclopedia of Mathematics and its Applications, vol. 48, Cambridge University Press, Cambridge, 1994.