Definition coprimep p q := size (gcdp p q) == 1%N.

Lemma Bezout_coprimepPn p q : p != 0 -> q != 0 ->
reflect (exists2 uv : {poly R} * {poly R},
(0 < size uv.1 < size q) && (0 < size uv.2 < size p) &
uv.1 * p = uv.2 * q)
(~~ (coprimep p q)).