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CHANGELOG.md

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The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
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and this project adheres to [Semantic Versioning v2.0.0](https://semver.org/spec/v2.0.0.html).
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## [1.0.1] - 2025-02-21
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### Changed
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- Update figure captions
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- Equation labeling change
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- Add coefficients U bibliography
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- Fix grammar abstract
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- Fix grammar introduction
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- Fix grammar generalizations
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- Fix grammar use cases
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- Fix grammar use conclusions
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- Update abstract
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## [1.0.0] - 2025-02-21
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mathematica/MathematicaPrograms.txt

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(* Requires package: PlotsOfClosedForms.m, see https://github.com/kolosovpetro/AnEfficientMethodOfSplineApproximation*)
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(* Requires package: PlotsOfClosedForms.m, see https://github.com/kolosovpetro/AnEfficientMethodOfSplineApproximation*)
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f1[X_] := X^7;
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f2[X_] := P[3, X, 50];

mathematica/figures_notebook.cdf

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out/AnEfficientMethodOfSplineApproximation.bbl

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\begin{thebibliography}{1}
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\begin{thebibliography}{10}
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\bibitem{alekseyev2018mathoverflow}
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{Alekseyev, Max}.
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\newblock
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\url{https://kolosovpetro.github.io/pdf/UnexpectedPolynomialIdentity.pdf}.
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\bibitem{oeis_coefficients_u_m_l_k_defined_by_polynomial_identity_1}
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Petro Kolosov.
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\newblock {The coefficients U(m, l, k), m = 1 defined by the polynomial
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identity}, 2018.
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\newblock \url{https://oeis.org/A320047}.
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\bibitem{oeis_coefficients_u_m_l_k_defined_by_polynomial_identity_2}
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Petro Kolosov.
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\newblock {The coefficients U(m, l, k), m = 2 defined by the polynomial
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identity}, 2018.
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\newblock \url{https://oeis.org/A316349}.
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\bibitem{oeis_coefficients_u_m_l_k_defined_by_polynomial_identity_3}
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Petro Kolosov.
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\newblock {The coefficients U(m, l, k), m = 3 defined by the polynomial
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identity}, 2018.
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\newblock \url{https://oeis.org/A316387}.
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\end{thebibliography}

out/AnEfficientMethodOfSplineApproximation.blg

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src/AnEfficientMethodOfSplineApproximation.tex

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\newtheorem{conjecture}[theorem]{Conjecture}
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\newtheorem{definition}[theorem]{Definition}
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\numberwithin{equation}{section}
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%\numberwithin{equation}{section}
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\title[An efficient method of spline approximation for power function]
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{An efficient method of spline approximation for power function}

src/sections/01_abstract.tex

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Let $P(m, X, N)$ be an $m$-degree polynomials in $X\in\mathbb{R}$
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having fixed non-negative integers $m$ and $N$.
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Essentially, the polynomial $P(m, X, N)$ is a result of rearrangement inside Faulhaber's formula
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in context of Knuth's work \textit{Johann Faulhaber and sums of powers}.
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In this manuscript we discuss approximation properties of polynomial $P(m,X,N)$.
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In particular, the polynomial $P(m,X,N)$ approximates odd power function $X^{2m+1}$ in certain neighborhood
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of fixed non-negative integer $N$ with percentage error lesser than $1\%$.
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By increasing the value of $N$ the length of convergence interval with odd-power $X^{2m+1}$ increasing as well.
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Furthermore, this approximation technique is generalized for arbitrary non-negative exponent power function $X^j$
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Essentially, the polynomial $P(m, X, N)$ is a result of a rearrangement inside Faulhaber's formula
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in the context of Knuth's work entitled "Johann Faulhaber and sums of powers".
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In this manuscript we discuss the approximation properties of polynomial $P(m,X,N)$.
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In particular, the polynomial $P(m,X,N)$ approximates the odd power function $X^{2m+1}$ in a certain neighborhood
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of a fixed non-negative integer $N$ with a percentage error less than $1\%$.
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By increasing the value of $N$ the length of convergence interval with odd-power $X^{2m+1}$ also increases.
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Furthermore, this approximation technique is generalized for arbitrary non-negative exponent of the power function $X^j$
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by using splines.

src/sections/02_introduction.tex

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For example,
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\input{sections/figures/05_fig_coefficients_a}
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Essentially, the polynomial $P(m, X, N)$ is a result of rearrangement inside Faulhaber's formula.
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Essentially, the polynomial $P(m, X, N)$ is derived from a rearrangement of Faulhaber's formula.
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It was inspired by Knuth's \textit{Johann Faulhaber and sums of powers}, see~\cite{knuth1993johann}.
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In particular, the polynomial $P(m, X, N)$ yields an identity for odd powers
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\begin{align*}
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P(m, X, X) = X^{2m+1}
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\end{align*}
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In extended form
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In its extended form
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\begin{align*}
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X^{2m+1} = \sum_{r=0}^{m} \sum_{k=1}^{X} \coeffA{m}{r} k^r (X-k)^r
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\end{align*}
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Precisely, the relation between Faulhaber's formula and $P(m,X,N)$ is shown by~\cite{kolosov2025unexpected}.
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The exact relation between Faulhaber's formula and $P(m,X,N)$ is shown by~\cite{kolosov2025unexpected}.
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However, apart polynomial identity for odd powers, I've spotted several approximation properties of $P(m,X,N)$.
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Therefore, in this manuscript we discuss approximation properties of polynomial $P(m,X,N)$.
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However, apart from the polynomial identity for odd powers, I've discovered several approximation properties of $P(m,X,N)$.
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Therefore, in this manuscript we explore the approximation properties of the polynomial $P(m,X,N)$.
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I use a few well-known criteria to measure and estimate error of approximation: Absolute error, Relative error and
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Percentage error.
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Assume that function $f_2(x)$ approximates the function $f_1 (x)$ then the errors are
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Assume that the function $f_2(x)$ approximates the function $f_1 (x)$, then errors are given by
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\begin{align*}
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\mathrm{Absolute \; Error} &= \lvert f_1(x) - f_2(x) \rvert \\
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\mathrm{Relative \; Error} &= \frac{\lvert f_1(x) - f_2(x) \rvert}{\lvert f_1(x) \rvert} \\
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\mathrm{Percentage \; Error} &= \frac{\lvert f_1(x) - f_2(x) \rvert}{\lvert f_1(x) \rvert} \times 100\%
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\end{align*}
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Diving straight to the point, we switch our focus to already mentioned polynomial $P(2,X,4) = 900X^2 - 6000X + 10624$
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Diving straight into the point, we switch our focus to the previously mentioned polynomial
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$P(2,X,4) = 900X^2 - 6000X + 10624$
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to show the first example of how it approximates the odd power function $X^5$.
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In fact, we approximate the polynomial $X^{2m+1}$ by lower degree polynomial $X^m$ as the following image presents
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In fact, we approximate the polynomial $X^{2m+1}$ using a lower-degree polynomial of degree $m$,
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as shown in the following image
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\begin{figure}[H]
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\centering
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\includegraphics[width=1\textwidth]{sections/images/03_plots_polynomial_p2_n4_with_fifth}
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~\caption{Polynomial plot $P(2, X, 4)$ with fifth power $X^5$.
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~\caption{Approximation of fifth power $X^5$ by $P(2, X, 4)$.
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Points of intersection $X=4$, $X=4.42472$, $X=4.99181$.
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Convergence interval: $4.0 \leq X \leq 5.1$ with percentage error $E < 1\%$.
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Convergence interval is $4.0 \leq X \leq 5.1$ with percentage error $E < 1\%$.
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}\label{fig:03_plots_polynomial_p2_n4_with_fifth}
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\end{figure}
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As we see, polynomial $P(2, X, 4)$ approximates $X^5$ in a neighborhood of $N=4$ with
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the convergence interval $4.0 \leq X \leq 5.1$ that has percentage error lesser than $1\%$ which is quite impressive.
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%Therefore, having fixed $N=4$ the polynomial $P(2, X, 4)$ approximates odd power in neighborhood of $N=4$
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%which is $3.9 \leq X \leq 5.3$.
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Consider the table below to showcase the concrete values of absolute, relative and percentage errors of this approximation
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As observed, the polynomial $P(2, X, 4)$ approximates $X^5$ in the neighborhood of $N=4$ with
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the convergence interval $4.0 \leq X \leq 5.1$ where the percentage error is less than $1\%$ which is quite remarkable.
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The following table presents specific values of absolute, relative, and percentage errors for this approximation
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\input{sections/figures/032_polynomials_p2_table_n4}
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One more interesting observation can be done by increasing the value of $N$ in $P(m, X, N)$ having fixed $m$, it
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follows that by increasing $N$ the length of convergence interval with odd-power $X^{2m+1}$ increasing as well.
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One more interesting observation arises by increasing the value of $N$ in $P(m, X, N)$ while keeping $m$ fixed.
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As $N$ increases, the length of the convergence interval with the odd-power $X^{2m+1}$ also increases.
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For instance,
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\begin{itemize}
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\item Having $P(2, X, 4)$ and $X^5$ the convergence interval with percentage error lesser than $1\%$ is $4.0 \leq X \leq 5.1$ with lenght of interval $L=1.1$
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\item Having $P(2, X, 20)$ and $X^5$ the convergence interval with percentage error lesser than $1\%$ is $18.7 \leq X \leq 22.9$ with lenght of interval $L=4.2$
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\item Having $P(2, X, 120)$ and $X^5$ the convergence interval with percentage error lesser than $1\%$ is $110.0 \leq X \leq 134.7$ with lenght of interval $L=24.7$
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\item For $P(2, X, 4)$ and $X^5$, the convergence interval with a percentage error less than $1\%$ is $4.0 \leq X \leq 5.1$, with a length $L=1.1$
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\item For $P(2, X, 20)$ and $X^5$, the convergence interval with a percentage error less than $1\%$ is $18.7 \leq X \leq 22.9$, with a length $L=4.2$
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\item For $P(2, X, 120)$ and $X^5$, the convergence interval with a percentage error less than $1\%$ is $110.0 \leq X \leq 134.7$, with a length $L=24.7$
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\end{itemize}
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The reason why the lenght of convergence interval rises as $N$ rise lays beneath the implicit form of polynomial $P(m,X,N)$
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The reason behind this behavior lies in the implicit form of the polynomial $P(m,X,N)$,
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meaning that
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\begin{align*}
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P(m,X,N) = \sum_{r=0}^{m} (-1)^{m-r} U(m, N, r) \cdot X^{r}
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\begin{align*}
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U(m, N, r) = (-1)^m \sum_{k=1}^{N} \sum_{j=r}^{m} \binom{j}{r} \coeffA{m}{j} k^{2j-r} (-1)^j
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\end{align*}
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which rises as $N$ rise.
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which grows as $N$ increases.
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Few cases of coefficients $U(m, N, r)$ are registered as OEIS sequences~\cite{
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oeis_coefficients_u_m_l_k_defined_by_polynomial_identity_1,
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oeis_coefficients_u_m_l_k_defined_by_polynomial_identity_2,
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oeis_coefficients_u_m_l_k_defined_by_polynomial_identity_3}.
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To wrap up the current state of the manuscript, refresh the key facts and finding we got so far.
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Therefore, the polynomial $P(m,X,N)$ is an $m$-degree polynomial in $X \in \mathbb{R}$, having fixed non-negative
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integers $m$ and $N$.
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It approximates odd power function $X^{2m+1}$ in some neighborhood of fixed $N$.
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The length $L$ of convergence interval between $X^{2m+1}$ and $P(m,X,N)$ rises as $N$ rise.
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To summarize, let us recap the key findings so far.
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The polynomial $P(m,X,N)$ is an $m$-degree polynomial in $X \in \mathbb{R}$ with fixed non-negative integers $m$ and $N$.
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It approximates the odd power function $X^{2m+1}$ within a specific neighborhood of $N$.
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The length $L$ of the convergence interval between $X^{2m+1}$ and $P(m,X,N)$ increases as $N$ grows.
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For the sake of clear and precise verification of results, I attach mathematica programs to generate
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plots and data tables, so that reader is able to verify the main results of current part of manuscript,
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\begin{figure}[H]
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\centering
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\includegraphics[width=1\textwidth]{sections/images/07_plot_of_6th_power_with_p_2_4_times_x}
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~\caption{Polynomial plot $P(2, X, 4)\cdot X$ with sixth power $X^6$.
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Convergence interval: $3.9 \leq X \leq 5.1$ with percentage error $E < 3\%$.
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~\caption{Approximation of sixth power $X^6$ by $P(2, X, 4) \cdot X$.
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Convergence interval is $3.9 \leq X \leq 5.1$ with percentage error $E < 3\%$.
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}\label{fig:07_plot_of_6th_power_with_p_2_4_times_x}
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\end{figure}
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Therefore, we have reached the statement that

src/sections/03_generalizations.tex

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Previously, we have discussed that polynomial $P(m,X,N)$ approximates power function $X^j$
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Previously, we have discussed that polynomial $P(m,X,N)$ approximates power function $X^j$
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in some neighborhood of fixed non-negative integer $N$.
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Approximation by $P(m,X,N)$ can be adjusted by $X^k$ multiplication for even exponents of power function.
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In general, it is safe to say that power function $X^j$ is approximated by $P(m,X,N) \cdot X^k$
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where $k=0$ for odd exponent $j$ and $k$ either $k=1$ or $k=-1$ for even exponent $j$.
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where $k=0$ for odd exponent $j$ and $k$ is either $k=1$ or $k=-1$ for an even exponent $j$.
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Therefore, for arbitrary exponent $j$ in $X^j$ we have
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\begin{align*}
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X^j \approx
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P(m,X,N) \cdot X^{-1} \quad & j=2m \\
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\end{cases}
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\end{align*}
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Of course, there are other variations of the value of $k$, we stick to simple case for the moment.
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Of course, there are other variations of the value of $k$, but we will stick to the simple case for the moment.
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As we also discussed, the lenght $L$ of convergence interval between $X^j$ and approximation by $P(m,X,N)$
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rises as $N$ rise.
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As we also discussed, the length $L$ of the convergence interval between $X^j$ and its approximation by $P(m,X,N)$
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increases as $N$ grow.
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However, the convergence interval is still bounded, which could not satisfy certain approximation scenarios.
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Depending on approximation requirements in terms of convergence interval length $L$ a single polynomial $P(m,X,N)$
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with fixed $m$ and $N$ can be unfit.
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Here is the place where spline approximation comes to play.
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Depending on the approximation requirements in terms of convergence interval length $L$ a single polynomial $P(m,X,N)$
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with fixed $m$ and $N$ may be unsuitable.
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Here is the place where spline approximation comes into play.
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The spline $S(x)$ is piecewise defined function over the interval $(x_0, \ldots x_n)$
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\begin{align*}
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S(x) &=
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\end{align*}
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The given points $x_k$ are called \textit{knots}.
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Assume that approximation requirement in terms of convergence length $L$ is to approximate the power function $X^j$
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Assume that the approximation requirement in terms of convergence length $L$ is to approximate the power function $X^j$
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bounded by real points $A$ and $B$ such that $A < B$.
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Splines perfectly fits the need to match arbitrary convergence range of power function's $X^j$
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Splines perfectly fit the need to match an arbitrary convergence range for the power function $X^j$ using the
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approximation by $P(m,X,N)$.
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Formally,
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\begin{align*}

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