> restart;
> unprotect(Psi):Psi[7]:=numtheory[cyclotomic](7,y);

                          6    5    4    3    2
               Psi[7] := y  + y  + y  + y  + y  + y + 1

> for i from 1 to 4 do
> Time := time():
> g := subs(y=x^(7^i),Psi[7]); print(g);
> print(`congruent  `, Factor(g) mod 2, `  mod 2`); print(`Time=` ,
> time()-Time); print(` `);
> od:

                  42    35    28    21    14    7
                 x   + x   + x   + x   + x   + x  + 1


                        21    14        21    7
         congruent  , (x   + x   + 1) (x   + x  + 1),   mod 2


                              Time=, 0.





               294    245    196    147    98    49
              x    + x    + x    + x    + x   + x   + 1


                      147    49        147    98
       congruent  , (x    + x   + 1) (x    + x   + 1),   mod 2


                             Time=, .101





            2058    1715    1372    1029    686    343
           x     + x     + x     + x     + x    + x    + 1


                    1029    686        1029    343
     congruent  , (x     + x    + 1) (x     + x    + 1),   mod 2


                             Time=, 6.920





          14406    12005    9604    7203    4802    2401
         x      + x      + x     + x     + x     + x     + 1


                   7203    4802        7203    2401
    congruent  , (x     + x     + 1) (x     + x     + 1),   mod 2


                           Time=, 2196.669




>