Sunday, 27 April 2014

Linear algebra question about definition of basis



From Wikipedia:



"A basis B of a vector space V over a field K is a linearly independent subset of V that spans (or generates) V.(1)



B is a minimal generating set of V, i.e., it is a generating set and no proper subset of B is also a generating set.(2)



B is a maximal set of linearly independent vectors, i.e., it is a linearly independent set but no other linearly independent set contains it as a proper subset."(3)




I tried to prove (1) => (3) => (2), to see that these are equivalent definitions, can you tell me if my proof is correct:



(1) => (3):
Let B be linearly independent and spanning. Then B is maximal: Let v be any vector in V. Then since B is spanning, biB,kiK:ni=1biki=v. Hence vni=1biki=0 and hence B{v} is linearly dependent. So B is maximal since v was arbitrary.



(3) => (2):
Let B be maximal and linearly independent. Then B is minimal and spanning:



spanning: Let vV be arbitrary. B is maximal hence B{v} is linearly dependent. i.e. biB,kiK:ibiki=v, i.e. B is spanning.




minimal: B is linearly independent. Let bB. Then bspan(B{b}) hence B is minimal.



(2) => (1):
Let B be minimal and spanning. Then B is linearly independent:
Assume B not linearly independent then biB,kiK:b=ibiki. Then B{b} is spanning which contradicts that B is minimal.


Answer



The proof looks good (appart form the obvious mix up in the numbering). One thing which is not totally precise:
In your second proof you write
Let vV be arbitrary. B is maximal hence B{v} is linearly dependent. i.e. biB,kiK:ibiki=v, i.e. B is spanning.

To be precise you have kiK not all vanishing such that k0v+ikibi=0. Since B is linearly independent k0=0 implies ki=0 for all i, therefore k00 and v is in the span of B.


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