Solution:
Define
fk(x)=n=0∑2k−1x+n(−1)s(n)
We want to show that f2022(2022)>0. We will in fact show something stronger.
I claim that for all x>0, for all k≥0, we have fk(i)(x)>0 for even i and fk(i)(x)<0 for odd i, where f(i) denotes the ith derivative of f. We will prove this claim with induction on k.
The base case of k=0 is easy to see because f0(x)=x1, so f0(2j)(x)=x2j+1(2j)!>0 and f0(2j−1)(x)=−x2j(2j−1)!<0 for all x>0.
Now, assume the claim is true for k=N. Then, note that
fN+1(x)=n=0∑2N+1−1x+n(−1)s(n)=n=0∑2N−1x+n(−1)s(n)+n=0∑2N−1x+n+2N(−1)s(n+2N)=n=0∑2N−1x+n(−1)s(n)−n=0∑2N−1x+n+2N(−1)s(n)=fN(x)−fN(x+2N)
Thus,
fN+1(2j)(x)=fN(2j)(x)−fN(2j)(x+2N)>0
since (fN(2j)(x))′=fN(2j+1)(x)<0. Similarly, we can show that fN+12j+1(x)<0, which completes the induction, so we are done.