finitely generated Module over a local ring and fibres of a vector space basis as generators

Proposition

Let 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_542d5d703f3c90578f829ea9bb609f758df8bb11.svg be a local ring with Residue field 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_c38d9e947cf51f4929894bfdb6df32852c1125fd.svg

Suppose 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_909cbe46a3cd8769d5e02ccd62f23447b7671461.svg is a finitely generated 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_4e4369590b92c3620f19dcd8077d03ff4203fbe1.svg-modules, 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_9d0dc04688a575784f8590d15c3d0a760e9548c6.svg the vector space over 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_e4b3d6f10424cbc18dcdc21d216c363a4ea05da1.svg (cf. functor from modules to vector spaces for the residue field).
Assume further that 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_e989d05df56cb5142ae1cc68e1fb141d5a12705b.svg generate 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_9d0dc04688a575784f8590d15c3d0a760e9548c6.svg as 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_fa58d7c11dd7499ea04324d7d87b62bf8af06035.svg module, and 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_2410258ef145e901690ffc7c4f26303a11cea8c9.svg are fibres of 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_ff616f9a856a1b092ee71fc8bdd9a2f96bc3d46a.svg.

Then 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_2410258ef145e901690ffc7c4f26303a11cea8c9.svg form a generating set

Proof

w.l.o.g. we may assume that 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_f7fd9b8e971c650b5d9313d0989ec322c2f01b3a.svg form a 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_fa58d7c11dd7499ea04324d7d87b62bf8af06035.svg-basis

Let 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_4da7f00cc03d8da4f0b8df6c9fa48038d7ecc692.svg be the generated submodule .
Then it remains to show that 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_8d2c598c5f69901ffb7b47e59fee1af2585e81d2.svg

consider the sequence of 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_542d5d703f3c90578f829ea9bb609f758df8bb11.svg modules

20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_ffdbd52ab4965e4966ce21b5b1d3158f899f42df.svg

it follows especially that the composition

20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_0286972f77653aaddc61c9b974ff2d9fa4a0bf91.svg

is surjective, as 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_9826b04be2eab0282b86142f2f658d94abc19a37.svg form a k-basis of 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_9d0dc04688a575784f8590d15c3d0a760e9548c6.svg and hence especially a generating set over 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_542d5d703f3c90578f829ea9bb609f758df8bb11.svg

using the first isomorphism theorem for modules there exists a diagram

20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_ce76feb90b7ca43cc0563c18f7c10c39a2f74e0c.svg

where

20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_66d22a2b99a312dbe596e93925cd19ec1f997f17.svg

and hence an isomorphism

20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_377f94e9788eb83d46de598991a11ba5d1ecea52.svg

This shows

20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_b4bb2bced01df92e54eccad12195ba6eb81ba01b.svg

by second Nakaymas Lemma for Jacobson Radical we conclude that 20240528-finitely_generated_module_over_a_local_ring_and_fibre_as_generators_4b746dc7d8b00c039fc81599f8fe7acc0aaf4d68.svg

Date: nil

Author: Anton Zakrewski

Created: 2026-06-22 Mo 10:14