Assume we have a Hida test function $\varphi\in (\mathcal S)$, and $y\in \mathcal S'(\mathbb R)$. Define the Gateaux directional derivative of $\varphi$ (in the direction of $y$) by:

$$D_y\varphi(x):=\sum_{n=1}^{\infty} n \langle :x ^{\otimes (n-1)}:,\langle y, f_n\rangle\rangle,$$

where the sequence of (symmetric) $f_n$'s is the element in the Fock space that corresponds to $\varphi$ (via the Ito-Segal isomorphism).

At this point I need to calculate the $S$-transform of this element, but I am having some problems (probably trivial stuff) with the calculations:

I have that by definition of the $S$-transform we have

$$S(D_y\varphi(x))(\xi)=\sum_{n=1}^{\infty} n\big\langle \langle y,f_n\rangle, \xi^{\otimes (n-1)}\big\rangle.$$

From this I should be able to obtain

$$\sum_{n=1}^{\infty} n\big\langle y\hat{\otimes} \xi^{\otimes (n-1)},f_n \big\rangle,$$

but this very last step is not clear to me and I haven't been able to find an explanation.

N.Obata justifies this step by using contraction tensors but prefer to avoid that formulation.

Could you please explain me this? I have the feeling that it's something straightforward but I haven't been able to get it.

Thanks in advance!