It's a follow up of Do we have $\prod_{n=1}^{\infty}\left(1+\frac{\tan\left(n\right)+1}{n\left(\tan\left(n+1\right)+1\right)}\right)=^?0$
Conjecture/Problem:
It seems we have for $N$ sufficiently large:
$$\sum_{n=1}^{N}\left|\left(1+\frac{\tan\left(n\right)+1}{n\left(\tan\left(n+1\right)+1\right)}\right)\right|-N-\ln^2(N)<0$$
Checking the sum until $N=800000$ there is no counter-example.
My progress:
I try to find a counter-example using the generalized triangle inequality we have:
$$\left|\sum_{n=1}^{N}\left(1+\frac{\tan\left(n\right)+1}{n\left(\tan\left(n+1\right)+1\right)}\right)\right|\leq \sum_{n=1}^{N}\left|\left(1+\frac{\tan\left(n\right)+1}{n\left(\tan\left(n+1\right)+1\right)}\right)\right|$$
So remains to find $M\geq N$ such that:
$$M+\ln^2(M)\leq \left|M+\sum_{n=1}^{M}\frac{\tan\left(n\right)+1}{n\left(\tan\left(n+1\right)+1\right)}\right|$$
Wich seems to be not true
On the other hand the function:
$$f(x)=\sum_{n=1}^{M}\left|1+\frac{\tan\left(n\right)+1}{xn\left(\tan\left(n+1\right)+1\right)}\right|$$
Seems convex on $(0,2)$ .
So keeping in mind this result (?) we can use a chord such that $x,a,b\in(0,2)$:
$$f(x)\leq \frac{f(a)-f(b)}{a-b}(x-a)+f(a)$$
But I cannot proceed further.
Edit :
It seems $\exists,n,m,M$ such that $m\leq n\leq M$ then we have :
$$\left|\frac{\tan(n)+1}{n\left(\tan\left(n+1\right)+1\right)}+1\right|-1-\frac{\ln^{2}\left(n\right)}{n}<0$$
Question:
How to (dis)prove it?
Any help is appreciated.