Subspace of P4: Polynomials of Even Degree

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SUMMARY

The set of polynomials of even degree within the vector space P4 is not a subspace. This conclusion is drawn from the fact that the zero polynomial, which is of degree 0, is not included in the set of polynomials of degree 2, thus violating the requirement for a subspace to contain the zero vector. Additionally, the sum of two polynomials of degree 2 can yield a polynomial of a different degree, further confirming that this set does not satisfy the closure properties necessary for a subspace.

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Homework Statement



Determine whether the following is a subspace of P_{4}_

(a) The set of polynomials in P_{4} of even degree.

Homework Equations



P_{4} = ax^{3}+bx^{2}+cx+d

The Attempt at a Solution



(p+q(x)) = p(x) + q(x)
(\alpha p)(x) = p(\alpha x)

If p and q are both of degree 2 then both scalar multiplication and vector addition should return a polynomial of degree 2 as far as I can tell, however my book states that this is not a subspace of P_{4}. I can't tell why it wouldn't be.
 
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The set of "polynomials of degree 2" are specifically those of the form ax2+ bx+ c with a non-zero. In particular, that does not include the 0 vector. Further, if p1(x)= x2+ 2x+ 1 and p2(x)= -ax2+ 3x+ 1, thenthe sum is NOT a "polynomial of degree 2". The polynomials in P4 that are of even degree are either of degree 2 or degree 0. In any case, the example I just gave answers your question.
 

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