theorem Th4:
  n>0 implies (n ^ (-(k+1)))=(n ^ (-k))/n
proof
  assume
A1: n>0;
  thus (n ^ (-(k+1))) = (n ^ ((-k)-1)) .= (n ^ (-k))/(n ^ 1) by A1,POWER:29
    .= (n ^ (-k))/n;
end;
