Zulip Chat Archive
Stream: mathlib4
Topic: inner_map_self_eq_zero
Iván Renison (Jun 23 2025 at 12:44):
Hi, in Analysis.InnerProductSpace.LinearMap we have this two theorems:
theorem inner_map_self_eq_zero (T : V →ₗ[ℂ] V) : (∀ x : V, ⟪T x, x⟫_ℂ = 0) ↔ T = 0
theorem ext_inner_map (S T : V →ₗ[ℂ] V) : (∀ x : V, ⟪S x, x⟫_ℂ = ⟪T x, x⟫_ℂ) ↔ S = T
I wanted to ask, do you know if there a reason they are for ℂ instead of for 𝕜 (with RCLike 𝕜)?
Sébastien Gouëzel (Jun 23 2025 at 12:48):
A rotation by in the plane sends every vector to an orthogonal vector, and yet it is non zero...
Iván Renison (Jun 23 2025 at 12:49):
Ah, okay, yes, thanks
Jireh Loreaux (Jun 23 2025 at 19:56):
Note, however, that it works for symmetric operators: docs#LinearMap.IsSymmetric.inner_map_self_eq_zero
Iván Renison (Jun 24 2025 at 07:01):
Great! Thanks
Iván Renison (Jun 24 2025 at 07:04):
And do you know if we have this versions?
lemma LinearMap.re_inner_eq_zero (T : E →ₗ[𝕜] E) :
(∀x y : E, RCLike.re ⟪T x, y⟫ = 0) ↔ T = 0 := sorry
lemma LinearMap.inner_eq_zero (T : E →ₗ[𝕜] E) :
(∀x y : E, ⟪T x, y⟫ = 0) ↔ T = 0 := sorry
Iván Renison (Jun 24 2025 at 07:16):
Also, in docs#LinearMap.IsSymmetric.inner_map_self_eq_zero I see that the description says if and only if `⟪T x, x⟫_ℝ = 0` , but the theorem uses inner 𝕜, is that an error?
I think both versions of the theorem are valid
Last updated: Dec 20 2025 at 21:32 UTC