I'm trying to see if there's a nice closed form expression for the following sum:
$\sum_{k=0}^{M} \cos(\pi k t) B_k(x)$
where $M \in \mathbb{N}$, $t \in (0,1)$, and $x \in \mathbb{R}^+$.
Notation: Here $B_{n}(x)$ denotes the $n$th Bell polynomial, $n \in \mathbb{N}$, of $x$, $x \in \mathbb{R}^+$, $S(n,k)$ denotes the Stirling number of the second kind.
From https://mathworld.wolfram.com/BellPolynomial.html (14),
$B_{n}(x) = \displaystyle{\sum_{k=0}^{n}} S(n,k) x^k$.
From https://mathworld.wolfram.com/StirlingNumberoftheSecondKind.html (10),
$S(n,k) = \frac{1}{k!} \displaystyle{\sum_{j=0}^{k} (-1)^j {k \choose j} (k-j)^n}$.
Truthfully my knowledge of these polynomials is limited to some informal skimming of the wikipedia and other pages/papers I could find, so I feel I am probably not taking the most strategic approaches in trying to solve this.
Some things I've attempted:
I've tried making use of Faà di Bruno's formula, to see if I could manipulate certain functions to produce the desired finite sum, but with no luck unfortunately.
Reference: https://en.wikipedia.org/wiki/Bell_polynomials#Applications
Same idea was tried with the generating functions of the Bell polynomials, but I can't seem to get it down.
Reference: https://en.wikipedia.org/wiki/Bell_polynomials#Generating_function .
I've also titled it a geometric like sum as it can also be viewed as $\Re[\sum_{k=0}^{M} (e^{i \pi t})^k B_k(x)]$.
I'd be curious to learn via any examples possibly provided of what would maybe be a better way to approach these types of problems in general for future reference! Would greatly appreciate any help. Thank you!