Morse index
A number associated with a critical point of a smooth function on a manifold or of a geodesic on a Riemannian (or Finsler) manifold.
1) The Morse index of a critical point of a smooth function f on a manifold M is equal, by definition, to the negative index of inertia of the Hessian of f at p ( cf. Hessian of a function), that is, the dimension of the maximal subspace of the tangent space T M _ {p} of M at p on which the Hessian is negative definite. This definition makes sense also for twice (Fréchet) differentiable functions on infinite-dimensional Banach spaces. The only difference is that the value + \infty is admissible for the index. In this case it is expedient to introduce the idea of the co-index of a critical point p of f as the positive index of inertia of the Hessian (the second Fréchet differential) of f at p .
2) Let V _ {0} and V _ {1} be smooth submanifolds of a complete Riemannian space M . For a piecewise-smooth path \omega : [ 0 , 1 ] \rightarrow M with \omega ( i) \in V _ {i} , i = 0 , 1 , transversal to V _ {0} and V _ {1} at its end-points \omega ( 0) and \omega ( 1) , the analogue of a tangent space is the vector space T _ \omega = T _ {\omega , V _ {0} , V _ {1} } of all piecewise-smooth vector fields W along \omega for which W ( \omega ( i) ) \in ( T V _ {i} ) _ {\omega ( i) } , i = 0 , 1 . For any geodesic \gamma : [ 0 , 1 ] \rightarrow M with \gamma ( i) \in V _ {i} , orthogonal at its end-points \gamma ( 0) and \gamma ( 1) to V _ {0} and V _ {1} , respectively, the second variation \delta ^ {2} E of the action functional (see Morse theory) defines a symmetric bilinear functional E _ {**} on T _ \gamma ( the analogue of the Hessian). The Morse index of the geodesic is equal, by definition, to the negative index of inertia of this functional. The null space N _ \gamma of E _ {**} on T _ \gamma ( the set of X \in T _ \gamma at which E _ {**} ( X , Y ) = 0 for all Y \in T _ \gamma ) consists exactly of the Jacobi fields (cf. Jacobi vector field) J \in T _ \gamma . If N _ \gamma \neq 0 , the geodesic is called ( V _ {0} , V _ {1} ) - degenerate, and \mathop{\rm dim} N _ \gamma is called the order of degeneracy of the geodesic.
The case when V _ {1} is a point q \in M is considered below. Let \nu be the normal bundle to V = V _ {0} in M and let \nu ( p) be its fibre over p \in V . The restriction of the exponential mapping T M \rightarrow M defines a mapping \mathop{\rm exp} : \nu \rightarrow M . A geodesic \gamma ( t) = \mathop{\rm exp} ( t \xi ) , \xi \in \nu ( p) , 0 \leq t \leq 1 , is ( V , \mathop{\rm exp} \xi ) - degenerate if and only if the kernel of the differential d _ \xi \mathop{\rm exp} : T \nu _ \xi \rightarrow T M _ { \mathop{\rm exp} \xi } of \mathop{\rm exp} at \xi is not null; in this connection, the dimension of the kernel is equal to the order of degeneracy of the geodesic \gamma . A point s = \gamma ( t _ {0} ) , 0 < t _ {0} \leq 1 , is called a focal point of V along \gamma if the geodesic \gamma ^ \prime : t \rightarrow \gamma ( t / t _ {0} ) is ( V , s ) - degenerate; the order of degeneracy of \gamma is called the multiplicity of the focal point s . By the Sard theorem, the set of focal points has measure zero, so a typical geodesic is non-degenerate. If V also consists of one point p \in M ( p = q is not excluded), then a focal point is called adjoint to p along \gamma . The Morse index theorem [1] asserts that the Morse index of a geodesic is finite and equal to the number of focal points \gamma ( t) of V , 0 < t < 1 , taking account of multiplicity.
References
[1] | M. Morse, "The calculus of variations in the large" , Amer. Math. Soc. (1934) |
[2] | W. Ambrose, "The index theorem in Riemannian geometry" Ann. of Math. , 73 (1961) pp. 49–86 |
Comments
There is a natural generalization of the Morse index of geodesics to variational calculus, which runs as follows. Let f be a real-valued smooth function on an open subset Z of [ 0 , 1 ] \times T M and let R be a smooth submanifold of M \times M . Let C be the space of smooth curves \omega : [ 0 , 1 ] \rightarrow M for which the 1 - jet lies in Z and ( \omega ( 0) , \omega ( 1) ) \in R . Then C is a Banach manifold, on which one has the smooth functional
F : \omega \mapsto \int\limits _ { 0 } ^ { 1 } f \left ( t , \omega ( t) , \frac{d \omega }{dt} ( t) \right ) d t .
One then considers the Morse index of F at critical curves \omega ; it is finite if the Hessian of v \mapsto f ( t , x , v ) is positive definite at x = \omega ( t) , v = ( d \omega / dt) ( t) , t \in [ 0 , 1 ] ( Legendre's condition, cf. Legendre condition).
References
[a1] | J.W. Milnor, "Morse theory" , Princeton Univ. Press (1963) |
[a2] | W. Klingenberg, "Riemannian geometry" , de Gruyter (1982) (Translated from German) |
[a3] | W. Klingenberg, "Lectures on closed geodesics" , Springer (1978) |
Morse index. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Morse_index&oldid=47903