Essential manifold

A closed manifold M is called essential if its fundamental class [M] defines a nonzero element in the homology of its fundamental group π, or more precisely in the homology of the corresponding Eilenberg–MacLane space K(π, 1), via the natural homomorphism

${\displaystyle H_{n}(M)\to H_{n}(K(\pi ,1)),}$

where n is the dimension of M. Here the fundamental class is taken in homology with integer coefficients if the manifold is orientable, and in coefficients modulo 2, otherwise.

• All closed surfaces (i.e. 2-dimensional manifolds) are essential with the exception of the 2-sphere S².
• Real projective space RPⁿ is essential since the inclusion

  ${\displaystyle \mathbb {RP} ^{n}\to \mathbb {RP} ^{\infty }}$

is injective in homology, where

${\displaystyle \mathbb {RP} ^{\infty }=K(\mathbb {Z} _{2},1)}$

is the Eilenberg–MacLane space of the finite cyclic group of order 2.

• All compact aspherical manifolds are essential (since being aspherical means the manifold itself is already a K(π, 1))
  • In particular all compact hyperbolic manifolds are essential.
• All lens spaces are essential.