/N 51 def
/k 3 def

1 setlinewidth

%% on genere un polygone regulier a N sommets
/les_points O 5.5 N 0 genpolyreg def

%% on relie le point d'indice i au point d'indice k*i (mod N)
/i 0 def
N {
   les_points i getp 
   les_points i k mul N mod getp
   1 trait
   /i i 1 add store
} repeat

%% la legende
/str 20 string def
setTimesItalic
   (n) xmax xmin 2 mul 3 div add .5 sub ymin text
setTimes
   ( = ) show
   N str cvs show
setTimesItalic
   (k) xmax xmin 4 mul 3 div add .5 sub ymin text
setTimes
   ( = ) show
   k str cvs show