# Show a function is linear

• PhysicsRock
In summary, the conversation is discussing how to approach and show that a function, specifically ##\partial##, is linear. The problem statement asks to prove that ##\partial## is a linear function for a given function ##f## and the notation ##j_K##. The conversation also addresses the requirements for a function to be linear and how to define the function ##\partial##. The conversation concludes with a summary of how ##\partial## transforms a given function.

#### PhysicsRock

Homework Statement
Take the Vector space of functions from a set ##\{ 0,1,...,n \}## to a field ##K##, ##V_n = \text{Fun}(\{ 0,1,...,n \},K)##. The notation ##j_K## refers to ##\underbrace{1_K + ... + 1_K}_{j \text{-times}}##. Let ##f \in V_{n+1}##. For a function ##\partial : V_{n+1} \rightarrow V_n## and ##f \in V_{n+1}##, show that ##\partial## is a linear function for ##\partial : i \rightarrow (i+1)_K \cdot f(i+1)##.
Relevant Equations
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I don't really know how I am supposed to approach that. In general, I know how to show that a function is linear, which is to show that ##f(\alpha \cdot x) = \alpha \cdot f(x)## and ##f(x_1 + x_2) = f(x_1) + f(x_2)##. However, for this specific function, I have no idea, since there is nothing provided about ##f##, so if I wanted to show the multiplicative property, I couldn't just drag anything out of ##f## without loss of generality. So I could really use some help to figure this out.

PhysicsRock said:
Homework Statement:: Take the Vector space of functions from a set ##\{ 0,1,...,n \}## to a field ##K##, ##V_n = \text{Fun}(\{ 0,1,...,n \},K)##. The notation ##j_K## refers to ##\underbrace{1_K + ... + 1_K}_{j \text{-times}}##. Let ##f \in V_{n+1}##. For a function ##\partial : V_{n+1} \rightarrow V_n## and ##f \in V_{n+1}##, show that ##\partial## is a linear function for ##\partial : i \rightarrow (i+1)_K \cdot f(i+1)##.
Relevant Equations::

Should this not be $$\partial(f) : i \mapsto (i+1)_K \cdot f(i+1)$$

EDIT: Also, I think we need $\partial: V_{n} \to V_{n+1}$ for $\partial(f)(n)$ to be defined.

I don't really know how I am supposed to approach that. In general, I know how to show that a function is linear, which is to show that ##f(\alpha \cdot x) = \alpha \cdot f(x)## and ##f(x_1 + x_2) = f(x_1) + f(x_2)##. However, for this specific function, I have no idea, since there is nothing provided about ##f##, so if I wanted to show the multiplicative property, I couldn't just drag anything out of ##f## without loss of generality. So I could really use some help to figure this out.

You are trying to show that $\partial : V_{n+1} \to V_n$ is linear, ie. $\partial(f_1 + f_2) = \partial (f_1) + \partial (f_2)$ for all $f_1, f_2 \in V_{n+1}$ and $\partial(\alpha f) = \alpha \partial (f)$ for every $\alpha \in K$ and every $f \in V_{n+1}$.

The operations on $V_{n+1}$ and $V_n$ are pointwise: $(f_1 + f_2)(x) = f_1(x) + f_2(x)$, $(\alpha f)(x) = \alpha f(x)$.

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• WWGD
pasmith said:
Should this not be $$\partial(f) : i \mapsto (i+1)_K \cdot f(i+1)$$

You are trying to show that $\partial : V_{n+1} \to V_n$ is linear, ie. $\partial(f_1 + f_2) = \partial (f_1) + \partial (f_2)$ for all $f_1, f_2 \in V_{n+1}$ and $\partial(\alpha f) = \alpha \partial (f)$ for every $\alpha \in K$ and every $f \in V_{n+1}$.

The operations on $V_{n+1}$ and $V_n$ are pointwise: $(f_1 + f_2)(x) = f_1(x) + f_2(x)$, $(\alpha f)(x) = \alpha f(x)$.
Yes, it's supposed to be ##\partial(f)##. Thank you, I forgot to state that. This helps.

pasmith said:
Should this not be $$\partial(f) : i \mapsto (i+1)_K \cdot f(i+1)$$

EDIT: Also, I think we need $\partial: V_{n} \to V_{n+1}$ for $\partial(f)(n)$ to be defined.

You are trying to show that $\partial : V_{n+1} \to V_n$ is linear, ie. $\partial(f_1 + f_2) = \partial (f_1) + \partial (f_2)$ for all $f_1, f_2 \in V_{n+1}$ and $\partial(\alpha f) = \alpha \partial (f)$ for every $\alpha \in K$ and every $f \in V_{n+1}$.

The operations on $V_{n+1}$ and $V_n$ are pointwise: $(f_1 + f_2)(x) = f_1(x) + f_2(x)$, $(\alpha f)(x) = \alpha f(x)$.
But one thing I don't understand: how do you know that ##f## obeys the requirements of a linear function?

PhysicsRock said:
But one thing I don't understand: how do you know that ##f## obeys the requirements of a linear function?

$f$ doesn't have to be linear; its domain is not even a vector space!

pasmith said:
EDIT: Also, I think we need ##\partial : V_n \rightarrow V_{n+1}## for ##\partial(f)(n)## to be defined.

I thought about that too. The assignment though says clearly what I stated in the problem description.

The problem statement asks you to show that $\partial$ is linear; it says nothing about $f$, other than that it is an arbitrary vector in $V_n$. Perhaps the fact that you used it in your statement of the definition of a linear function is confusing you.

pasmith said:
The problem statement asks you to show that $\partial$ is linear; it says nothing about $f$, other than that it is an arbitrary vector in $V_n$. Perhaps the fact that you used it in your statement of the definition of a linear function is confusing you.
I pretty much only translated the assignment. I felt like including everything was helpful to receive the needed aid.

PhysicsRock said:
I pretty much only translated the assignment. I felt like including everything was helpful to receive the needed aid.
When you have something as abstract as this, you need to get a grip on what everything is.

I would have called ##\partial## an operator that maps functions in ##V_{n+1}## to functions in ##V_n##.

For ##f \in V_{n+1}##, what does ##d = \partial(f)## look like?

We know that ##d \in V_n## must be defined for ##i = 0, 1 \dots n##. Working from the definition:
$$d(0) = 1_Kf(1)$$$$d(1) = 2_Kf(2)$$$$\dots d(n) = (n+1)_Kf(n+1)$$Now that we see how ##\partial## transforms functions the fun can begin!

Let ##f, g \in V_{n+1}## and ##a \in K##. We need to show that:
$$\partial(f +g) = \partial(f) + \partial(g)$$$$\partial(af) = a\partial(f)$$Perhaps that's more help than I should have given you. It seems to me that the material you're studying exceeds your current capability to think in terms of abstract mathematics.