Euler's Integral Theorem/Proof 1

Proof
Recall the definition of the floor function:

For all $n \in \N_{>0}$:

Let $N \ge n \ge 1$.

By Sum of Integrals on Adjacent Intervals for Integrable Functions:
 * $\ds (1): \quad \paren {H_n - \ln n} - \paren {H_N - \ln N} = \int_n^N \dfrac {u - \floor u} {u^2} \rd u$

On the other hand, from follows:
 * $\forall u \in \R_{\ge 1} : 0 \le \dfrac {u - \floor u} {u^2} \le \dfrac 1 {u^2}$

In view of Integral Operator is Positive, integrating the above inequality on $\closedint n N$:

From $(1)$ and $(2)$, it follows:
 * $(3): \quad 0 \le \paren {H_n - \ln n} - \paren {H_N - \ln N} \le \dfrac 1 n$

In particular, $\sequence {H_n - \ln n}$ is a Cauchy sequence.

Thus the limit $\gamma$, the Euler-Mascheroni constant, exists by Cauchy's Convergence Criterion.

In $(3)$, for each $n \in \N$, let $N \to \infty$.

Then:
 * $\forall n \in \N : 0 \le H_n - \ln n - \gamma \le \dfrac 1 n$