diff --git a/chapter12.tex b/chapter12.tex
index 90ae9c4..fe77bac 100644
--- a/chapter12.tex
+++ b/chapter12.tex
@@ -546,4 +546,4 @@ it is difficult to find out if the nodes
 in a graph can be colored using $k$ colors
 so that no adjacent nodes have the same color.
 Even when $k=3$, no efficient algorithm is known
-but the problem is NP-hard \cite{gar79}.
\ No newline at end of file
+but the problem is NP-hard.
\ No newline at end of file
diff --git a/chapter17.tex b/chapter17.tex
index 8dbe992..ba99272 100644
--- a/chapter17.tex
+++ b/chapter17.tex
@@ -559,9 +559,5 @@ and both $x_i$ and $x_j$ become false.
 A more difficult problem is the \key{3SAT problem}
 where each part of the formula is of the form
 $(a_i \lor b_i \lor c_i)$.
-This problem is NP-hard \cite{gar79}, so no efficient algorithm
-for solving the problem is known.
-
-
-
-
+This problem is NP-hard, so no efficient algorithm
+for solving the problem is known.
\ No newline at end of file