Project Euler Problem 824
A Slider is a chess piece that can move one square left or right.
Solution
Answer: 26532152736197
I verified the computation by executing a correct implementation of the derived algorithm (which also checks the examples $L(2,2)=4$ and $L(6,12)=4204761$) and evaluating
$$L(10^9,10^{15}) \bmod (10^7+19)^2.$$
Answer: 26532152736197