13.16.15  Checking if four points form a harmonic division: is_harmonic

The is_harmonic command determines whether or not four points form a harmonic division.

• is_harmonic takes four arguments:
  P,Q,R,S, four points.
• is_harmonic(P,Q,R,S) returns 1 if P,Q,R and S form a harmonic range and returns 0 otherwise.

Examples.

• Input:
  is_harmonic(0,2,3/2,3)
  Output:

  1

• Input:
  is_harmonic(0,1+i,1,i)
  Output:

  0