Proof with auto with typeclass_instances.
intros x b Hb0 Hxb.
destruct (Req_dec x 0) as [Hx0|Hx0].
(* *)
exists 0%R.
split.
now rewrite Rabs_R0.
rewrite Hx0, Rmult_0_l.
apply round_0...
(* *)
exists ((round beta fexp rnd x - x) / x)%R.
split. 2: now field.
unfold Rdiv.
rewrite Rabs_mult.
apply Rmult_le_reg_r with (Rabs x).
now apply Rabs_pos_lt.
rewrite Rmult_assoc, <- Rabs_mult.
rewrite Rinv_l with (1 := Hx0).
now rewrite Rabs_R1, Rmult_1_r.
Qed.
End relative_error_conversion.
Variable emin p : Z.
Hypothesis Hmin : forall k, (emin < k)%Z -> (p <= k - fexp k)%Z.
Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.

Theorem relative_error :
forall x,
(bpow emin <= Rabs x)%R ->
(Rabs (round beta fexp rnd x - x) < bpow (-p + 1) * Rabs x)%R.