Implement infectivity radius on Stiefel (#722)

* Implement injectivity radius on Stiefel --------- Co-authored-by: Mateusz Baran <[email protected]>
JuliaManifolds · May 7, 2024 · bd0264f · bd0264f · kellertuer · May 7, 2024
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diff --git a/NEWS.md b/NEWS.md
@@ -5,7 +5,14 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [0.9.17] – unreleased
+## [0.9.18] – 2024-05-07
+
+### Added
+
+* added the injectivity radius for the Stiefel manifold with Euclidean metric
+
+
+## [0.9.17] – 2024-04-23
 
 ### Added
 

diff --git a/Project.toml b/Project.toml
@@ -1,7 +1,7 @@
 name = "Manifolds"
 uuid = "1cead3c2-87b3-11e9-0ccd-23c62b72b94e"
 authors = ["Seth Axen <[email protected]>", "Mateusz Baran <[email protected]>", "Ronny Bergmann <[email protected]>", "Antoine Levitt <[email protected]>"]
-version = "0.9.17"
+version = "0.9.18"
 
 [deps]
 Distributions = "31c24e10-a181-5473-b8eb-7969acd0382f"

diff --git a/docs/src/references.bib b/docs/src/references.bib
@@ -880,6 +880,15 @@ @article{ZimmermannHueper:2022
     EPRINT     = {2103.12046},
     EPRINTTYPE = {arXiv}
 }
+@article{ZimmermannStoye:2024,
+    AUTHOR     = {Zimmermann, Ralf and Stoye, Jakob},
+    EPRINT     = {2405.02268},
+    EPRINTTYPE = {arXiv},
+    JOURNAL    = {arXiv Preprint},
+    TITLE      = {The injectivity radius of the compact Stiefel manifold under the Euclidean metric},
+    URL        = {https://arxiv.org/abs/2405.02268},
+    YEAR       = {2024}
+}
 #
 # Å
 # ----------------------------------------------------------------------------------------

diff --git a/src/manifolds/StiefelEuclideanMetric.jl b/src/manifolds/StiefelEuclideanMetric.jl
@@ -102,7 +102,15 @@ end
 
 _euclidean_unit_vector(n, i) = [k == i ? 1.0 : 0.0 for k in 1:n]
 
+@doc raw"""
+    injectivity_radius(M::Stiefel)
+
+Return the injectivity radius for the [`Stiefel`](@ref) manifold `M`,
+which is globally ``π`` [ZimmermannStoye:2024](@cite).
 """
+injectivity_radius(::Stiefel) = π
+
+@doc raw"""
     inverse_retract(M::Stiefel, p, q, method::ProjectionInverseRetraction)
 
 Compute a projection-based inverse retraction.

diff --git a/test/manifolds/stiefel.jl b/test/manifolds/stiefel.jl
@@ -10,6 +10,8 @@ include("../header.jl")
             @test is_default_metric(M, EuclideanMetric())
             @test representation_size(M) == (3, 2)
             @test manifold_dimension(M) == 3
+            @test injectivity_radius(M) == π
+            @test injectivity_radius(M2) == π
             @test !is_flat(M)
             @test !is_flat(M2)
             @test is_flat(Stiefel(2, 1))