adding reference

Author: Sandstorm831

src/doc/en/reference/references/index.rst

@@ -3821,6 +3821,10 @@ REFERENCES:
              *Bethe ansatz and inverse scattering transform in a periodic
              box-ball system*, Nuclear Phys. B **747**, no. 3 (2006), 354--397.
 
+.. [KTZ1987] Kierstead, H.A., Trotter, W.T. & Zhou, B. Representing an ordered
+             set as the intersection of super greedy linear extensions. Order 4,
+             293-311 (1987). 
+             :doi:`10.1007/BF00337892`
 .. [Kuh1987] \W. Kühnel, "Minimal triangulations of Kummer varieties",
              Abh. Math. Sem. Univ. Hamburg 57 (1987), 7-20.
 

src/sage/combinat/posets/linear_extensions.py

@@ -268,7 +268,7 @@ def is_supergreedy(self):
         Informally, a linear extension is supergreedy if it "always
         goes up and receedes the least"; in other words, supergreedy
         linear extensions are depth-first linear extensions.
-        `Reference <https://link.springer.com/article/10.1007/BF00337892>`_
+        For more details see [KTZ1987]_.
 
         EXAMPLES::