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Conjugacy is a property of two points independent of the geodesic between them
Statement
False claim: Whether two endpoints are conjugate is independent of the specified geodesic segment joining them. On the unit round sphere , the point is conjugate to itself along a full great-circle loop, but is not conjugate to itself along the constant segment.
Facts & Assumptions
Given: The standard unit round sphere and the explicit parameter interval .
If are orthonormal, then is an affinely parametrized geodesic; constant curves are also geodesics (Great circles as round-sphere geodesics).
For a smooth variation through affinely parametrized geodesics, its variation field is a Jacobi field along the central geodesic (Variation field of a geodesic variation is a Jacobi field).
For a specified geodesic segment with , the endpoints are conjugate along exactly when there is a nonzero Jacobi field vanishing at both endpoints (Conjugate points along a geodesic and their multiplicity).
If the specified segment is constant, its endpoint-vanishing Jacobi-field space is , so its endpoints are not conjugate (Conjugate points along a geodesic and their multiplicity).
Refutation
Let , , , and for ; then are orthonormal for each . [construct, algebra] 2.1 Define on . Orthonormality and give , so is a smooth map into ; by [F1], each longitudinal curve is a great-circle affine geodesic. Thus is a geodesic variation. [F1, step 1.1, algebra] 3.1 The central geodesic has , and differentiating gives , with and . By [F2], is Jacobi; [F3] makes the endpoints conjugate along this full loop. [F2, F3, step 2.1, algebra] 4.1 On the same interval let . By [F1] it is a constant geodesic with the same endpoints as , but [F4] says those endpoints are not conjugate along ; hence this endpoint pair has different conjugacy outcomes along the two segments. [F1, F4, step 3.1] 5.1 Since , the witness interval is nondegenerate; is smooth up to both included endpoints and has the displayed endpoint zeros, while its nonzero value in step 3.1 witnesses conjugacy. The fixed example needs no dimension-zero or dimension-one case, and its standard basis and variation are explicit, so neither AC nor is used; no iff is asserted.
Source locator
Lee, Riemannian Manifolds, Proposition 5.13 and its complete proof, printed pp.82-83 / PDF labels P98-99, lines 3427-3466, identifies round-sphere geodesics as great circles. The library's Great circles as round-sphere geodesics derives the explicit all-time formula used here. Lee's Lemma 10.8 and proof, printed pp.179-180 / PDF labels P195-196, lines 6974-7020, reduces normal Jacobi fields on a constant-curvature geodesic to the sine equation. Lee's Chapter 10 definition and Proposition 10.11, printed pp.182-183 / PDF labels P198-199, lines 7169-7250, define conjugacy along a specified geodesic and relate it to geodesic variations. Those passages do not compare the full-loop segment with the constant segment; the explicit field and constant-geodesic conclusion above are established from the cited library items.
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Sources
- John M. Lee, Riemannian Manifolds: An Introduction to Curvature (1997) (standard reference, not scraped)
- John M. Lee, Riemannian Manifolds: An Introduction to Curvature (1997) (standard reference, not scraped)
- John M. Lee, Riemannian Manifolds: An Introduction to Curvature (1997) (standard reference, not scraped)