- #1
thelema418
- 132
- 4
I'm confused by a set of problems my teacher created versus a set of problems in the textbook.
My textbook states that "A vector space V over a field F consists of a set on which two operations (called addition and scalar multiplication, respectively) are defined so that for each pair of elements, x, y in V there is a unique element x + y in V, and for each element a in F and each element x in V there is a unique element ax in V, such that the following conditions hold..."
In our problem set, the professor created a problem where V is over ##\mathbb{R}^+##, but the scalars are members of ##\mathbb{R}##. There is a special definition of addition and scalar multiplication, so I can easily prove that the axioms hold.
My concern is with the use of two different fields. I'm thinking that this is not a vector space because of the field membership.
The issue I'm having with the textbook problem is that a true or false makes the claim "If f is a polynomial of degree #n# and #c# is a nonzero scalar, then #cf# is a polynomial of degree #n#." If I follow the textbooks definition the way I'm interpreting it, I get the answer True. But if I use two different fields, the answer will be false. For example, let the polynomials be from ##Z_3(x)## but the scalars from ##Z_7## and defined multiplication and addition with mod 3. The #c = 6# is a nonzero scalar, but (6 * 2x) mod 3 = 0.
In short, I cannot reconcile what the professor is doing in his question with what the definition appears to say to me. Is there something I'm not seeing?
My textbook states that "A vector space V over a field F consists of a set on which two operations (called addition and scalar multiplication, respectively) are defined so that for each pair of elements, x, y in V there is a unique element x + y in V, and for each element a in F and each element x in V there is a unique element ax in V, such that the following conditions hold..."
In our problem set, the professor created a problem where V is over ##\mathbb{R}^+##, but the scalars are members of ##\mathbb{R}##. There is a special definition of addition and scalar multiplication, so I can easily prove that the axioms hold.
My concern is with the use of two different fields. I'm thinking that this is not a vector space because of the field membership.
The issue I'm having with the textbook problem is that a true or false makes the claim "If f is a polynomial of degree #n# and #c# is a nonzero scalar, then #cf# is a polynomial of degree #n#." If I follow the textbooks definition the way I'm interpreting it, I get the answer True. But if I use two different fields, the answer will be false. For example, let the polynomials be from ##Z_3(x)## but the scalars from ##Z_7## and defined multiplication and addition with mod 3. The #c = 6# is a nonzero scalar, but (6 * 2x) mod 3 = 0.
In short, I cannot reconcile what the professor is doing in his question with what the definition appears to say to me. Is there something I'm not seeing?