Zero is overrated

Define a sequence of functions \(P_i:\mathbb{R}^+\rightarrow\mathbb{R}\), \(i\in\mathbb{N}\)

$$P_0(x) = \ln(x)$$ $$P_{n+1}(x) = \int P_{n}(x)\cdot dx$$

I found a beautiful closed formula for \(P_i\), that I haven't seen before.

Integrating by parts, it's easy to calculate first few \(P_i\):

$$\begin{array}{r@{\;}c@{\;}l@{\quad}} P_1(x) &\;=\;& x\cdot\ln x - x \\ P_2(x) &\;=\;& \frac{x^2}{2}\cdot\ln x - \frac{3}{4}\cdot x^2 \\ P_3(x) &\;=\;& \frac{x^6}{6}\cdot\ln x - \frac{11}{36}\cdot x^3 \end{array}$$

which leads to a hypothesis, that

$$P_n = \frac{x^n}{n!}\cdot\ln x - K_n\cdot x^n$$

for certain constants \(K_i\), \(K_1 = 1\).

Again integrating by parts, obtain:

$$K_{n+1} = \frac{1}{n+1}\cdot\Bigg(K_n + \frac{1}{(n+1)!}\Bigg)$$

from where

$$\begin{array}{r@{\;}c@{\;}l@{\quad}} K_n &\;=\;& \frac{1}{n}\cdot\Bigg(K_{n-1} + \frac{1}{n!}\Bigg) \\ &\;=\;& \frac{1}{n}\cdot\Bigg(\frac{1}{n!} + \frac{1}{n-1}\cdot\big(K_{n-2} + \frac{1}{(n-1)!}\big)\Bigg) \\ &\;=\;& \frac{1}{n!}\cdot\Bigg(\frac{1}{n} + \frac{1}{n-1}\Bigg) + \frac{1}{n\cdot(n - 1)}\cdot K_{n-2} \\ &\;=\;& \frac{1}{n!}\cdot\Bigg(\frac{1}{n} + \frac{1}{n - 1} + \frac{1}{n - 2}\Bigg) + \frac{1}{n\cdot(n-1)\cdot(n - 2)}\cdot K_{n-3} \\ &\;=\;& \cdots \\ &\;=\;& \frac{1}{n!}\cdot\Bigg(\frac{1}{n} + \frac{1}{n - 1} + \cdots +\frac{1}{n - p + 1}\Bigg) + \frac{1}{n\cdot(n-1)\cdot\cdots\cdot(n - p +1)}\cdot K_{n-p} \end{array}$$

Substitute \(p = n - 1\):

$$K_n = \frac{1}{n!}\cdot\Bigg(1 + \frac{1}{2} + \cdots + \frac{1}{n}\Bigg)$$

Substitute this in the hypothesis:

$$P_n = \frac{x^n}{n!}\cdot\Bigg(\ln x - \big(1 + \frac{1}{2} + \cdots + \frac{1}{n}\big)\Bigg)$$

This nicely contains fragments of exponent, nth-harmonic number and, after a diagonalisation, the Euler constant:

$$\lim_{n\to +\infty}\frac{n!}{n^n}\cdot P_n(n) = -\gamma$$

Why \(P_i\) are interesting at all? Because if one completes them for negative indices as

$$P_n = (-1)^{n-1}\cdot(-n-1)!\cdot x^n$$

then mth-derivative of \(P_n\) is \(P_{n-m}\) for all non-negative \(m\):

$$(\forall n\in\mathbb{Z})(\forall m\in\mathbb{N})\partial^m P_n = P_{n - m}$$

and similarly

$$(\forall n\in\mathbb{Z})(\forall m\in\mathbb{N})\int_m P_n\cdot dx = P_{n + m}$$

where \(\int_m\) is repeated integral.

This is in contrast with powers \(x^n\), \(n \ge 0\), which, under repeated derivation, eventually pathetically collapse to a constant and then to 0, so that negative powers are not reachable from positive and other way around.

It's interesting, which other families \((\phi_i)_{i\in\mathbb{Z}}\) are there such that

$$(\forall m\in\mathbb{N})\partial^m\phi_n = \phi_{n - m}$$ $$(\forall m\in\mathbb{N})\int_m \phi_n\cdot dx = \phi_{n + m}$$

and

$$(\forall n\neq m)\phi_n \neq const\cdot\phi_m$$

(the latter condition is to avoid degenerate cases)?