Group Homomorphism/Examples/Pointwise Addition on Continuous Real Functions on Closed Unit Interval/Example 5

Example of Group Homomorphism
Let $J \subseteq \R$ denote the closed unit interval $\closedint 0 1$.

Let $\map {\mathscr C} J$ denote the set of all continuous real functions from $J$ to $\R$.

Let $G = \struct {\map {\mathscr C} J, +}$ be the group formed on $\map {\mathscr C} J$ by pointwise addition.

Let $\struct {\R, +}$ denote the additive group of real numbers.

From Pointwise Addition on Continuous Real Functions on Closed Unit Interval forms Group we have that $G$ is indeed a group.

Let $\phi: \struct {\map {\mathscr C} J, +} \to \struct {\R, +}$ be the mapping defined as:
 * $\forall f \in \map {\mathscr C} J: \map \phi f = \ds \int_0^1 \map \cos {\dfrac {\pi \map f x} 6} \rd x$

Then $\phi$ is not a homomorphism.

Proof
Let $f, g \in \map {\mathscr C} J$ be arbitrary.

As both $f$ and $g$ are continuous real functions on $J$, both $\map \cos {\dfrac {\pi \map f x} 6}$ and $\map \cos {\dfrac {\pi \map g x} 6}$ are integrable on $\closedint 0 1$.

We have:

However:

Thus:
 * $\map \phi {f + g} \ne \map \phi f + \map \phi g$

and so $\phi$ is not a homomorphism by definition.