10 0. MORSE THEORY AND VARIATIONAL PROBLEMS
It was also shown in Perera  that the assumptions that B is bounded
and Φ is bounded from below on bounded sets can be relaxed as follows; see
also Schechter .
Theorem 0.26. If A homologically links B in dimension k, Φ|A ď a ă Φ|B
where a is a regular value, and Φ is bounded from below on a set C Ą B
such that the inclusion-induced homomorphism
H kpW zCq Ñ
H kpW zBq is
trivial, then Φ has two critical points u1 and u2 with
Φpu1q ą a ą Φpu2q, Ck`1pΦ,u1q ‰ 0, CkpΦ,u2q ‰ 0.
Corollary 0.27. Let W “ W1 ‘ W2, u “ u1 ` u2 be a direct sum decompo-
sition with dim W1 “ k ă 8. If Φ ď a on u1 P W1 : }u1} “ R
R ą 0, Φ ą a on W2, where a is a regular value, and Φ is bounded from
below on tv ` u2 : t ě 0, u2 P W2
for some v P W1z t0u, then Φ has two
critical points u1 and u2 with
Φpu1q ą a ą Φpu2q, CkpΦ,u1q ‰ 0, Ck´1pΦ,u2q ‰ 0.
The following theorem of Perera and Schechter  gives a critical point
with a nontrivial critical group in a saddle point theorem with nonstandard
geometrical assumptions that do not involve a finite dimensional closed loop;
see also Perera and Schechter  and Lancelotti .
Theorem 0.28. Let W “ W1 ‘ W2, u “ u1 ` u2 be a direct sum decompo-
sition with dim W1 “ k ă 8. If Φ is bounded from above on W1 and from
below on W2, then Φ has a critical point u1 with
inf ΦpW2q ď Φpu1q ď sup ΦpW1q, CkpΦ,u1q ‰ 0.
0.6. Local Linking
In many applications Φ has the trivial critical point u “ 0 and we are
interested in finding others. The notion of local linking was introduced by
Li and Liu [72, 66], who used it to obtain nontrivial critical points under
various assumptions on the behavior of Φ at infinity; see also Brezis and
Nirenberg  and Li and Willem .
Definition 0.29. Assume that the origin is a critical point of Φ with Φp0q “
0. We say that Φ has a local linking near the origin if there is a direct sum
decomposition W “ W1 ‘ W2, u “ u1 ` u2 with W1 finite dimensional such
Φpu1q ď 0, u1 P W1, }u1} ď r
Φpu2q ą 0, u2 P W2, 0 ă }u2} ď r
for suﬃciently small r ą 0.
Liu  showed that this yields a nontrivial critical group at the origin.
Theorem 0.30. If Φ has a local linking near the origin with dim W1 “ k
and the origin is an isolated critical point, then CkpΦ, 0q ‰ 0.