% 6.1; equilibria, stability.
 % 6.2; phase portraits for non-linear systems, Paste Theorem
 % 6.3; populations and ecological models, predator-prey, competition, survival, extinction
\begin{alltt}
HW 12, due Week 13
 \BF{5.6} [5.7 in edition 5/E]: 1, 13, \bx{15}, 19,  \bx{23}
 \BF{6.1}: 3, \bx{4}, 5, 7, \bx{8}, 11, 15, 17, \bx{18}
 \BF{6.2}: \bx{2}, 5, 7, 9, \bx{12}, 13, 17, 19, 21, \bx{22}, 23, 29
 \BF{6.3}: 5, 7, \bx{8}, \bx{9}, \bx{10}, 11, 18, 19
\end{alltt}