diff --git a/CHANGELOG_UNRELEASED.md b/CHANGELOG_UNRELEASED.md
index 607e14f95..2dcf3d137 100644
--- a/CHANGELOG_UNRELEASED.md
+++ b/CHANGELOG_UNRELEASED.md
@@ -81,6 +81,9 @@
   + lemma `lim_lime_inf`
   + lemma `lim_lime_sup`
 
+- in `normedtype.v`:
+  + lemmas `withinN`, `at_rightN`, `at_leftN`, `cvg_at_leftNP`, `cvg_at_rightNP`
+
 ### Changed
 
 - in `normedtype.v`:
diff --git a/theories/normedtype.v b/theories/normedtype.v
index e60aacb08..2375669ba 100644
--- a/theories/normedtype.v
+++ b/theories/normedtype.v
@@ -2087,6 +2087,30 @@ apply: contrapT; apply: pePf; apply/andP; split.
 - by near: t; apply: nbhs_right_lt; rewrite ltr_addl.
 Unshelve. all: by end_near. Qed.
 
+Lemma withinN (A : set R) a :
+  within A (nbhs (- a)) = - x @[x --> within (-%R @` A) (nbhs a)].
+Proof.
+rewrite eqEsubset /=; split; move=> E /= [e e0 aeE]; exists e => //.
+  move=> r are ra; apply: aeE; last by rewrite memNE opprK.
+  by rewrite /= opprK addrC distrC.
+move=> r aer ar; rewrite -(opprK r); apply: aeE; last by rewrite -memNE.
+by rewrite /= opprK -normrN opprD.
+Qed.
+
+Lemma at_rightN a : (- a)^'+ = -%R @ a^'-.
+Proof.
+rewrite /at_right withinN (_ : [set - x | x in _] = (fun u => u < a))//.
+apply/seteqP; split=> [x [r /[1!ltr_oppl] ? <-//]|x xa/=].
+by exists (- x); rewrite 1?ltr_oppr ?opprK.
+Qed.
+
+Lemma at_leftN a : (- a)^'- = -%R @ a^'+.
+Proof.
+rewrite /at_left withinN (_ : [set - x | x in _] = (fun u => a < u))//.
+apply/seteqP; split=> [x [r /[1!ltr_oppr] ? <-//]|x xa/=].
+by exists (- x); rewrite 1?ltr_oppr ?opprK.
+Qed.
+
 End at_left_right.
 #[global] Typeclasses Opaque at_left at_right.
 Notation "x ^'-" := (at_left x) : classical_set_scope.
@@ -2098,6 +2122,20 @@ Notation "x ^'+" := (at_right x) : classical_set_scope.
 #[global] Hint Extern 0 (Filter (nbhs _^'-)) =>
   (apply: at_left_proper_filter) : typeclass_instances.
 
+Lemma cvg_at_leftNP {T : topologicalType} {R : numFieldType}
+    (f : R -> T) a (l : T) :
+  f @ a^'- --> l <-> f \o -%R @ (- a)^'+ --> l.
+Proof.
+by rewrite at_rightN -?fmap_comp; under [_ \o _]eq_fun => ? do rewrite /= opprK.
+Qed.
+
+Lemma cvg_at_rightNP {T : topologicalType} {R : numFieldType}
+    (f : R -> T) a (l : T) :
+  f @ a^'+ --> l <-> f \o -%R @ (- a)^'- --> l.
+Proof.
+by rewrite at_leftN -?fmap_comp; under [_ \o _]eq_fun => ? do rewrite /= opprK.
+Qed.
+
 Section open_itv_subset.
 Context {R : realType}.
 Variables (A : set R) (x : R).
diff --git a/theories/realfun.v b/theories/realfun.v
index f47a784a4..9fb711502 100644
--- a/theories/realfun.v
+++ b/theories/realfun.v
@@ -152,38 +152,14 @@ Lemma cvg_at_leftP (f : R -> R) (p l : R) :
   (forall u : R^nat, (forall n, u n < p) /\ (u --> p) ->
     f (u n) @[n --> \oo] --> l).
 Proof.
-split=> [/cvgrPdist_le fpl u [up /cvgrPdist_lt ucvg]|pfl].
-  apply/cvgrPdist_le => e e0.
-  have [r /= r0 {}fpl] := fpl _ e0; have [s /= s0 {}ucvg] := ucvg _ r0.
-  near=> t; apply: fpl => //=; apply: ucvg => /=.
-  by near: t; exists s.
-apply: contrapT => fpl; move: pfl; apply/existsNP.
-suff: exists2 x : R ^nat,
-    ((forall k, x k < p) /\ x --> p) & ~ f (x n) @[n --> \oo] --> l.
-  by move=> [x_] h; exists x_; apply/not_implyP.
-have [e He] : exists e : {posnum R}, forall d : {posnum R},
-    exists xn : R, [/\ xn < p, `|xn - p| < d%:num & `|f xn - l| >= e%:num].
-  apply: contrapT; apply: contra_not fpl => /forallNP h.
-  apply/cvgrPdist_le => e e0; have /existsNP[d] := h (PosNum e0).
-  move/forallNP => {}h; near=> t.
-  have /not_and3P[abs|abs|/negP] := h t.
-  - by exfalso; apply: abs; near: t; exact: nbhs_left_lt.
-  - exfalso; apply: abs.
-    by near: t; exists d%:num => //= z/=; rewrite distrC.
-  - by rewrite -ltNge distrC => /ltW.
-have invn n : 0 < n.+1%:R^-1 :> R by rewrite invr_gt0.
-exists (fun n => sval (cid (He (PosNum (invn n))))).
-  split => [k|]; first by rewrite /sval/=; case: cid => x [].
-  apply/cvgrPdist_lt => r r0; near=> t.
-  rewrite /sval/=; case: cid => x [px xpt _].
-  rewrite distrC (lt_le_trans xpt)// -(@invrK _ r) lef_pinv ?posrE ?invr_gt0//.
-  near: t; exists `|ceil (r^-1)|%N => // s /=.
-  rewrite -ltnS -(@ltr_nat R) => /ltW; apply: le_trans.
-  by rewrite natr_absz gtr0_norm ?ceil_gt0 ?invr_gt0// ceil_ge.
-move=> /cvgrPdist_lt/(_ e%:num (ltac:(by [])))[] n _ /(_ _ (leqnn _)).
-rewrite /sval/=; case: cid => // x [px xpn].
-by rewrite leNgt distrC => /negP.
-Unshelve. all: by end_near. Qed.
+apply: (iff_trans (cvg_at_leftNP f p l)).
+apply: (iff_trans (cvg_at_rightP _ _ _)).
+split=> [pfl u [pu up]|pfl u [pu up]].
+  rewrite -(opprK u); apply: pfl.
+  by split; [move=> k; rewrite ltr_oppr opprK//|exact/cvgNP].
+apply: pfl.
+by split; [move=> k; rewrite ltr_oppl//|apply/cvgNP => /=; rewrite opprK].
+Qed.
 
 End cvgr_fun_cvg_seq.