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answered 2017-02-15 10:40:00 +0100
You should evaluate D at point (f,m):
D
(f,m)
sage: [(f,m) for m in [-10..10] for f in [-10..10] if D(f,m)==0] [(-1, -2), (1, -2), (-1, 0), (-1, 2), (1, 2)]