changed [Read1968] to [Rea1968], added [Rea1968]_ under chromatic_polynomial, changed the reference to previous one

Author: Krishnadubey1008

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

@@ -5644,6 +5644,9 @@ REFERENCES:
               Congressus numerantium, 1994.
               Pages 97--110
 
+.. [Rea1968] Ronald C. Read, An improved method for computing the chromatic polynomials of
+             sparse graphs.
+
 .. [Rea2004] Nathan Reading.
              *Cambrian Lattices*.
              :arxiv:`math/0402086v2`.
@@ -5686,10 +5689,6 @@ REFERENCES:
              `Downloadable from Reutenauer's website
              <http://www.lacim.uqam.ca/~christo/Publi%C3%A9s/2003/free%20Lie%20algebras.pdf>`_
 
-.. [Read1968] Ronald C. Read, *An Introduction to Chromatic Polynomials*,
-              Journal of Combinatorial Theory 4 (1968), 52-71.
-              :doi:`10.1016/S0021-9800(68)80087-0`.
-
 .. [Rho69] John Rhodes, *Characters and complexity of finite semigroups*
            \J. Combinatorial Theory, vol 6, 1969
 

src/sage/graphs/chrompoly.pyx

@@ -10,7 +10,7 @@ AUTHORS:
 
 REFERENCE:
 
-    See [Read1968]_ and the :wikipedia:`Chromatic_polynomial` for more details on this notion in graphs.
+    See [Rea1968]_ and the :wikipedia:`Chromatic_polynomial` for more details on this notion in graphs.
 
 """
 
@@ -41,7 +41,7 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
     Compute the chromatic polynomial of the graph G.
 
     The algorithm used is a recursive one, based on the following observations
-    of Read:
+    of Read [Rea1968]_:
 
         - The chromatic polynomial of a tree on n vertices is x(x-1)^(n-1).