\textbf{Claim 1.} ##\text{Sym}(\varphi)## is a linear map from ##V \to W##.
Proof:
First, we have ##\text{char} \mathbb{K} \not\vert \vert G \vert##. So, ##\vert G \vert \neq 0## and ##\frac{1}{\vert G \vert}## is defined. For each ##g \in G##, we have ##(\tau(g) \circ \varphi \circ \rho(g^{-1})(v) \in W##. Since ##W## is a vector space, it is closed under addition and scalar multiplication. So, ##(\text{Sym}\varphi)(v) \in W##. Next, we check that ##\text{Sym}(\varphi)## is ##\mathbb{K}##-linear. Let ##u, v \in V## and ##\lambda \in \mathbb{K}##. We have
\begin{align*}
(\text{Sym}\varphi)(u + v) & = \left(\frac{1}{\vert G \vert} \sum_{g \in G} \tau(g) \circ \varphi \circ \rho(g^{-1})\right) (u + v) \\
&= \frac{1}{\vert G \vert} \sum_{g \in G} \left(\tau(g) \circ \varphi \circ \rho(g^{-1}) (u + v)\right) \\
&= \frac{1}{\vert G \vert} \sum_{g \in G} \left(\tau(g) \circ \varphi \circ (\rho(g^{-1})(u) + \rho(g^{-1})(v))\right) \\
&= \frac{1}{\vert G \vert} \sum_{g \in G} \left(\tau(g) \circ (\varphi \circ \rho(g^{-1})(u) + \varphi \circ \rho(g^{-1})(v))\right) \\
&= \frac{1}{\vert G \vert} \sum_{g \in G} (\tau(g) \circ \varphi \circ \rho(g^{-1})(u)) + (\tau(g) \circ + \varphi \circ \rho(g^{-1})(v)) \\
&= \frac{1}{\vert G \vert} \sum_{g \in G}\tau(g) \circ \varphi \circ \rho(g^{-1})(u) + \frac{1}{\vert G \vert} \sum_{g \in G}\tau(g) \circ + \varphi \circ \rho(g^{-1})(v) \\
&= (\text{Sym}\varphi)(u) + (\text{Sym}\varphi)(v)\\
\end{align*}
Also,
\begin{align*}
(\text{Sym}\varphi)(\lambda v) &= \frac{1}{\vert G \vert} \sum_{g \in G} (\tau(g) \circ \varphi \circ \rho(g^{-1}))(\lambda v) \\
&= \frac{1}{\vert G \vert} \sum_{g \in G} \lambda \left(\tau(g) \circ \varphi \circ \rho(g^{-1})(v)\right) \\
&= \lambda \left(\frac{1}{\vert G \vert} \sum_{g \in G} (\tau(g) \circ \varphi \circ \rho(g^{-1}))(v)\right) \\
&= \lambda (\text{Sym}\varphi)(v) \\
\end{align*}
This shows ##\text{Sym}\varphi## is ##\mathbb{K}##-linear.
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\textbf{Claim 2.} The map ##\text{Sym}## is a ##\mathbb{K}##-linear mapping.
Proof:
Let ##\varphi, \sigma \in \text{Hom}_{\mathbb{K}}(V, W)## and ##\lambda \in \mathbb{K}##. For any ##g \in G##, we have
\begin{align*}
\text{Sym}(\varphi + \sigma) &= \frac{1}{\vert G \vert}\sum_{g \in G}\tau(g) \circ (\varphi + \sigma) \circ \rho(g^{-1}) \\
&= \frac{1}{\vert G \vert}\sum_{g \in G}\tau(g) \circ ((\varphi \circ \rho(g^{-1}) + (\sigma \circ \rho(g^{-1}))) \\
&=\frac{1}{\vert G \vert}\sum_{g \in G} (\tau(g) \circ \varphi \circ \rho(g^{-1})) + (\tau(g) \circ \sigma \circ \rho(g^{-1})) \\
&=\frac{1}{\vert G \vert}\sum_{g \in G} (\tau(g) \circ \varphi \circ \rho(g^{-1})) + \frac{1}{\vert G \vert}\sum_{g \in G} (\tau(g) \circ \sigma \circ \rho(g^{-1})) \\
&= \text{Sym}(\varphi) + \text{Sym}(\sigma)
\end{align*}Also,
\begin{align*}
\text{Sym}(\lambda\varphi) &= \frac{1}{\vert G \vert}\sum_{g \in G} \tau(g) \circ (\lambda\varphi) \circ \rho(g^{-1}) \\
&= \frac{1}{\vert G \vert}\sum_{g \in G} k(\tau(g) \circ \varphi \circ \rho(g^{-1})) \\
&= k\frac{1}{\vert G \vert}\sum_{g \in G} \tau(g) \circ \varphi \circ \rho(g^{-1}) \\
&= k \text{Sym}(\varphi) \\
\end{align*}
We may conclude ##\text{Sym}## is ##\mathbb{K}##-linear.
[]
\textbf{Claim 3.} ##\text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W)) = \lbrace \theta : V \longrightarrow W \vert \forall_{g \in G} \tau(g) \circ \theta \circ \rho(g^{-1}) = \theta\rbrace##
Proof:
##(\subseteq)##: Let ##\theta \in \text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))##. By definition, we have ##\theta \circ \rho(g) = \tau(g) \circ \theta## for all ##g \in G##. In particular, ##\tau(g) \circ \theta \circ \rho(g^{-1}) = \theta \circ \rho(g) \circ \rho(g^{-1}) = \theta##.
\\
##(\supseteq)##: Suppose for all ##g \in G##, ##\tau(g) \circ \theta \circ \rho(g^{-1}) = \theta##. Multiplying both sides by ##\rho(g)##, we have ##\tau(g) \circ \theta = \theta \circ \rho(g)##. This shows ##\supseteq##.
[]\textbf{Claim 4.} ##\text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))## is a subspace of ##\text{Hom}_{\mathbb{K}}(V, W)##.
Proof:
Consider the map that sends every element in ##V## to ##0##. This map is contained in ##\text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))##. So, ##\emptyset \neq \text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W)) \subset \text{Hom}_{\mathbb{K}}(V,W)##. Let ##\varphi, \sigma \in \text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W)), \lambda \in \mathbb{K}## and ##g \in G##.
\\
We have $$(\varphi + \sigma)\circ \rho(g) = (\varphi \circ \rho(g)) + (\sigma\circ \rho(g)) = (\tau(g) \circ \varphi) + (\tau(g) \circ \sigma) = \tau(g)(\varphi + \sigma)$$
So, ##\varphi + \sigma \in \text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))##. Also,
$$((\lambda \varphi) \circ \rho(g)) = \lambda(\varphi \circ \rho(g)) = \lambda (\tau(g) \circ \varphi) = (\lambda\tau(g)) \circ \varphi = \tau(g) \circ (\lambda \varphi)$$
So, ##\lambda \varphi \in \text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))##. We can conclude ##\text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))## is a subspace of ##\text{Hom}_{\mathbb{K}}(V, W)##.
[]
\textbf{Claim 5.} ##\text{Sym}## is a projection onto ##\text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))##.
Proof:
Let ##\varphi \in \text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))##. Then ##\text{Sym}(\varphi) = \varphi##. In particular, ##\text{Sym}(\text{Sym}(\varphi)) = \text{Sym}(\varphi)##. It follows that ##\text{Sym}^2 = \text{Sym}## on ##\text{Hom}_{\mathbb{K}}((\rho, V), (\tau, W))##. This proves Claim 5.
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