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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 S2, the point p=(1,0,0) 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 S2⊂R3 and the explicit parameter interval [0,2π].

[F1]

If p,u∈S2 are orthonormal, then t⟼cos⁡(t)p+sin⁡(t)u is an affinely parametrized geodesic; constant curves are also geodesics (Great circles as round-sphere geodesics).

[F2]

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).

[F3]

For a specified geodesic segment η:[a,b]→M with a<b, the endpoints are conjugate along η exactly when there is a nonzero Jacobi field vanishing at both endpoints (Conjugate points along a geodesic and their multiplicity).

[F4]

If the specified segment η is constant, its endpoint-vanishing Jacobi-field space is {0}, so its endpoints are not conjugate (Conjugate points along a geodesic and their multiplicity).

Refutation

1.1F1F2F3F4step 1.1step 2.1step 3.1step 4.1∎

Let p=e0=(1,0,0), e1=(0,1,0), e2=(0,0,1), and u(s)=cos⁡(s)e1+sin⁡(s)e2 for −1<s<1; then p,u(s) are orthonormal for each s. [construct, algebra] 2.1 Define F(s,t)=cos⁡(t)e0+sin⁡(t)u(s) on (−1,1)×[0,2π]. Orthonormality and cos⁡2t+sin⁡2t=1 give ∣F(s,t)∣=1, so F is a smooth map into S2; by [F1], each longitudinal curve is a great-circle affine geodesic. Thus F is a geodesic variation. [F1, step 1.1, algebra] 3.1 The central geodesic γ(t)=F(0,t)=cos⁡(t)e0+sin⁡(t)e1 has γ(0)=γ(2π)=p, and differentiating gives J(t)=∂sF(0,t)=sin⁡(t)e2, with J(0)=J(2π)=0 and J(π/2)=e2≠0. By [F2], J is Jacobi; [F3] makes the endpoints conjugate along this full loop. [F2, F3, step 2.1, algebra] 4.1 On the same interval let δ(t)=p. By [F1] it is a constant geodesic with the same endpoints as γ, but [F4] says those endpoints are not conjugate along δ; hence this endpoint pair has different conjugacy outcomes along the two segments. [F1, F4, step 3.1] 5.1 Since 0<2π, the witness interval is nondegenerate; J 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 S2 example needs no dimension-zero or dimension-one case, and its standard basis and variation are explicit, so neither AC nor ACω 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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