Hello there Mathematicians!
Recently I was playing around with some numbers and I stumbled across this mathematical expressionthe following formal power series:
$$\sum_{k=0}^\infty\frac{x^{ak}}{(ak)!}\biggl(\sum_{l=0}^k\binom{ak}{al}\biggr)$$
I was able to solve this equation"simplify" the above expression for $a=1$:
$$\sum_{k=0}^\infty\frac{x^k}{k!}\cdot2^k=e^{2x}$$ I also managed to solvesimplify the equationexpression for $a=2$ with the identity $\sum_{i=0}^\infty\frac{x^{2k}}{(2k)!}=\cosh(x)$:
$$\sum_{k=0}^\infty\frac{x^{2k}}{(2k)!}\biggl(\sum_{l=0}^k\binom{2k}{2l}\biggr)=\mathbf[\cdots\mathbf]=\frac{1}{4}\cdot(e^{2x}+e^{-2x})+\frac{1}{2}=\frac{1}{2}\cdot(\cosh(2x)+1)$$
However, I couldn't come up with a general solutionmethod for all $a\in\Bbb{N}$. I would be very thankful if someone could either guide me towards solvingsimplifying this equationexpression or post his solution here. Thank you very much in advance!