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  1. MHB Master
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    #1
    Hey!!

    Let $a\in \mathbb{R}$. We define the map $\text{cost}_a:\mathbb{R}\rightarrow \mathbb{R}$, $x\mapsto a$. We define also $-f:=(-1)f$ for a map $f:\mathbb{R}\rightarrow \mathbb{R}$.

    Let $f:\mathbb{R}\rightarrow\mathbb{R}$ be a map and $\lambda\in \mathbb{R}$.

    Show that:

    1. for $a,b\in \mathbb{R}$ it holds that $\text{cost}_a+\text{cost}_b=\text{cost}_{a+b}$.
    2. for $a\in \mathbb{R}$ it holds that $\lambda\text{cost}_a=\text{cost}_{\lambda a}$.
    3. $-(f+g)=(-f)+(-g)$.
    4. $f+f=2f$.
    5. $f+(-f)=\text{cost}_0$.


    Could you give me a hint how we could these? Aren't all of these trivial?

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  3. MHB Seeker
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    #2
    Hey mathmari!!

    I guess the first one can be shown as follows:
    $$\DeclareMathOperator{\cost}{cost}
    \forall a,b\in\mathbb R,\,\forall x\in\mathbb R : (\cost_a+\cost_b)(x)=\cost_a(x)+\cost_b(x)=a+b=\cost_{a+b}(x)$$
    Therefore:
    $$\forall a,b\in\mathbb R : \cost_a+\cost_b=\cost_{a+b}$$


    And yes, it does look rather trivial.

  4. MHB Master
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    #3 Thread Author
    Quote Originally Posted by Klaas van Aarsen View Post
    I guess the first one can be shown as follows:
    $$\DeclareMathOperator{\cost}{cost}
    \forall a,b\in\mathbb R,\,\forall x\in\mathbb R : (\cost_a+\cost_b)(x)=\cost_a(x)+\cost_b(x)=a+b=\cost_{a+b}(x)$$
    Therefore:
    $$\forall a,b\in\mathbb R : \cost_a+\cost_b=\cost_{a+b}$$
    Ahh ok!!

    In the same way we could show also the other ones, or not?

    $$2. \ \ \ \DeclareMathOperator{\cost}{cost}
    \forall a\in\mathbb R,\,\forall x\in\mathbb R : \lambda \cost_a(x)=\lambda a=\cost_{\lambda a}(x)$$
    Therefore:
    $$\forall a\in\mathbb R : \lambda\cost_a=\cost_{\lambda a}$$

    $$3. \ \ \ \forall x\in\mathbb R : -(f+g)(x)=(-1)(f+g)(x)=(-1)(f(x)+g(x))=(-1)f(x)+(-1)g(x)=(-f)(x)+(-g)(x)$$
    Therefore:
    $$-(f+g)=(-f)+(-g)$$

    $$4. \ \ \ \forall x\in\mathbb R : (f+f)(x)=f(x)+f(x)=2f(x)$$
    Therefore:
    $$f+f=2f$$

    $$5. \ \ \ \forall x\in\mathbb R : (f+(-f))(x)=(f+(-1)f)(x)=f(x)+(-1)f(x)=(1-1)f(x)=0\cdot f(x)=0=\text{cost}_0(x)$$
    Therefore:
    $$f+(-f)=\text{cost}_0$$


    Is everything correct? Could we improve something?

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    #4
    Looks all good to me.

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    #5 Thread Author
    Quote Originally Posted by Klaas van Aarsen View Post
    Looks all good to me.
    Great!! Thank you!!

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