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[] V) : ( x : V, T x, x⟫_ = 0)  T = 0
theorem ext_inner_map (S T : V →ₗ[] V) : ( x : V, S x, x⟫_ = T x, x⟫_)  S = T

I wanted to ask, do you know if there a reason they are for instead of for 𝕜 (with RCLike 𝕜)?

Sébastien Gouëzel (Jun 23 2025 at 12:48):

A rotation by π/2\pi/2 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