 3 ) list all subdivision trees of N (enumeration problem); and 4 ) find a subdivision tree to maximize or minimize a prescribed objective function (optimization problem).
<meaning-changed>  3 ) list all subdivision trees of N (enumeration problem); and 4 ) find a subdivision tree to maximize or minimize a prescribed objective function (optimization problem).
 3 ) list all subdivision trees of N ; and 5 ) find a subdivision tree to maximize or minimize a prescribed objective function (optimization problem).