Scientific Computing

The following section presupposes Set Theory and Nonstandard Analysis. Let \(\varepsilon \in [\tilde{\nu}, 1 – \tilde{\nu}]\).

Definition: Let \(f_n^*(z) = f(\eta_nz)\) sisters of the TS \(f(z) \in \mathcal{O}(D)\) centred on 0 on the domain \(D \subseteq {}^{\omega}\mathbb{C}\) where \(m, n \in {}^{\omega}\mathbb{N}^{*}\) and \(\eta_n^m := \underline{1}^{2^{\lceil m/n \rceil}}\). Then let \(\delta_n^*f = \check{f} – \check{f}_n^*\) the halved sister distances of \(f\). Let \(\mu_n^m := m!n!/(m + n)!\) and \(\widetilde{(-n)!} := 0\). For \(j, k \in {}^{\omega}\mathbb{N}, f \in \mathcal{C}_{\pi}^{j+2}\) and Fourier coefficients \(c_k := \tilde{\hat{\pi}}{\uparrow}_{-\pi}^{\pi}{f(t)\tilde{\epsilon}^{\underline{k}t}{{\downarrow}t}}\)¹cf. Walter, Wolfgang: Analysis 2; 5., erw. Aufl.; 2002; Springer; Berlin, p. 358 ff., the series \({\LARGE{\textbf{+}}}_{k=-\omega}^{\omega}{c_k{\epsilon}^{\underline{k}t}}\) is called Fourier series. On the level of TS, \(\mu\) and \(\eta\) form a \(\mu\)-\(\eta\)-calculus.\(\triangle\)

Remark: The latter allows representing integrals and derivatives easily and finitely closed²cf. Remmert, Reinhold: Funktionentheorie 1; 3., verb. Aufl.; 1992; Springer; Berlin, p. 165 f.. A conversion to other period lengths than \(\hat{\pi }\) is simply possible³see loc. cit., p. 364.

Example: \(\text{Re} \; c \in [\tilde{\nu}, 1 + \tilde{\nu}[, c \in {}^{\omega}\mathbb{C}\) and \(\varepsilon := \tilde{2}^j, j \in {}^{\omega}\mathbb{N}^{*}\) imply \(\zeta (n+c)=a_n{\LARGE{\textbf{+}}}_{k=1}^{n}{\delta_n^* v_c(\varepsilon u^k)}\) for \(z \in {}^{1-\tilde{\nu}}\dot{\mathbb{C}} \subset \mathbb{D}\)⁴cf. Remmert, Reinhold: Funktionentheorie 2; 1. unveränd. Nachdruck der 1. Aufl.; 1992; Springer; Berlin, p. 42 and \(v_c(z):={\LARGE{\textbf{+}}}_{m=1}^{\omega }{\zeta (m+c){{z}^{m}}}=z{\LARGE{\textbf{+}}}_{m=1}^{\omega }{{{{\widetilde{m}}}^{c}}\widetilde{z-m}}.\square\)

Example: A Richardson extrapolation determines the digamma function \(\psi\) and\[\zeta(\grave{n}) =\widetilde{\varepsilon^n\hat{n}}{\LARGE{\textbf{+}}}_{k=1}^n{(\psi(\varepsilon u^k \underline{1}^{2\tilde{n}}) – \psi(\varepsilon u^k))} + \mathcal{O}(\varepsilon^{\hat{n}})\] for \(n = 2\) and \(\varepsilon = \tilde{2}^{13}\) as \(\zeta(3) = 2^{24}{\LARGE{\textbf{+}}}_{k=1}^2{(\psi(\underline{\varepsilon} u^k) – \psi(\varepsilon u^k))} + \mathcal{O}(10^{-16})\).

Representation theorem for integrals: The TS (see below) \(f(z) \in \mathcal{O}(D)\) centred on 0 for \(D \subseteq {}^{\omega}\mathbb{C}\) gives for \(\grave{m}, n \in {}^{\omega}\mathbb{N}^*\)\[{\uparrow}_0^z…{\uparrow}_0^{\zeta_2}{f(\zeta_1){\downarrow}\zeta_1\;…\;{\downarrow}\zeta_n} = \widetilde{n!} f(z\mu_n) z^n.\square\]Example: For the TS \(f(x), g(x) \in {}^{\omega}\mathbb{R}\), it holds that \({\uparrow}_0^x{f(v){\downarrow}v}{\uparrow}_0^x{\uparrow}_0^{y}{g(v){\downarrow}v{\downarrow}y} = f(x\mu_1)\check{g}(x\mu_2)x^3.\)

Representation theorem for derivatives: For \({}^{\widetilde{\nu}}\dot{\mathbb{C}} \subset \mathbb{D} \subseteq {}^{\omega}\mathbb{C}, n\)-th unit roots, TS\[f(z):=f(0) + {\LARGE{\textbf{+}}}_{m=1}^{\omega }{\widetilde{m!}\ {}^mf(0){z^m}},\]\(\varepsilon := \tilde{2}^j\tilde{r}, j \in {}^{\omega}\mathbb{Z}, n = {\epsilon}^{\sigma} \in {}^{\omega}\mathbb{N}^{*}, u :={\epsilon}^{\tilde{n} \hat{\underline{\pi}}}\) and \(f\)’s radius of convergence \(r \in {}^{\nu}{\mathbb{R}}_{>0}\) imply\[{}^nf(0)=2^{jn}\acute{n}!{\LARGE{\textbf{+}}}_{k=1}^{n}{\delta_n^* f(\tilde{2}^j u^k)}.\square\]Universal multistep theorem: For \(n \in {}^{\nu}\mathbb{N}_{\le p}, k, m, p \in {}^{\nu}\mathbb{N}^{*}, {\downarrow}\overset{\rightharpoonup}{x} \in\, ]0, 1[, x \in [a, b] \subseteq {}^{\omega}\mathbb{R}, y : [a, b] \rightarrow {}^{\omega}\mathbb{R}^q,\) \(f : [a, b]\times{}^{\omega}\mathbb{R}^{q \times n} \rightarrow {}^{\omega}\mathbb{R}^q, g_k(\overset{\rightharpoonup}{x}) := g_{\acute{k}}(x)\), and \(g_0(a) = f(\overset{\leftharpoonup}{a}, y_0, … , y_{\acute{n}})\), the TS of the initial value problem \(y^\prime(x) = f(x, y((\rightharpoonup)^0 x), … , y((\rightharpoonup)^{\acute{n}} x))\) of order \(n\) implies\[y(\overset{\rightharpoonup}{x}) = y(x) + {\downarrow}\overset{\rightharpoonup}{x}{\LARGE{\textbf{$\pm$}}}_{k=1}^{p}{\left (g_{p-k}(\overset{\rightharpoonup}{x}){\LARGE{\textbf{+}}}_{m=k}^{p}{\widetilde{m!}\tbinom{\acute{m}}{\acute{k}}}\right )} + \mathcal{O}(({\downarrow}\overset{\rightharpoonup}{x})^{\grave{p}}).\square\]Theorem for (anti-) derivatives of TS: For \(m \in {}^{\omega}\mathbb{Z}\), \(q = \tilde{\varepsilon}(z-a)\), \(a \in \mathbb{D}\) and \(j, k \in \mathbb{N}_{<n}\), modular arithmetic⁵cf. Knuth, Donald Ervin: The Art of Computer Programming Volume 2; 3rd Ed.; 1997; Addison Wesley; Reading, p. 302 – 311 and \(n\)-th unit roots result in the corresponding DFT form:\[{}^mf_n(z) := \tilde{n}(q^j)^T(\delta_{jk}\widetilde{\varepsilon q}^jj!/(j-m)!)({\tilde{u}}^{jk})(f({\varepsilon u}^k+a))+\mathcal{O}(\varepsilon^n).\square\]Reversion theorem of TS: For \(y \in f(\mathbb{D}), y(a) = b\) and \({}^1y(a) \ne 0\), Bürmann’s theorem yields⁶see Whittaker, Edmund T.; Watson, George N.: A Course of Modern Analysis; 5th Ed.; 2021; Cambridge University Press; Cambridge, p. 129 ff. :\[f^{-1}(y) = a + \tilde{n} {\LARGE{\textbf{+}}}_{m=1}^n{\widetilde{m}{\tilde{\varepsilon}}^{\acute{m}}(y – b)^m({\tilde{u}}^{\acute{m}k})^T(f(\varepsilon u^k + a)^{-m})}+\mathcal{O}(\varepsilon^n).\square\]Remark: Put \(y = 0\) for a sufficiently small \(|a – z_0|\) such that \(z_0 \in {}^{\omega}\mathbb{C}\) is a zero of \(f(z) \in {}^{\omega}\mathbb{C}\).

Theorem for derivatives of Fourier series: For \(f \in \mathcal{C}_{\pi}^{m+2}\)⁷cf. Walter, loc. cit., \(m \in {}^{\omega}\mathbb{N}, j \in [-\acute{n}, n] \cap \mathbb{Z}, t \in [-\pi, \pi]\) and \(k \in \mathbb{N}_{<\hat{n}}\), the following DFT form is implied allowing integration of individual terms:\[{}^mf_n(t):=\tilde{n}(u^{\tilde{\pi}jnt})^T(\delta_{(j+\acute{n})k}\underline{j}^m)(\tilde{u}^{jk})(\check{f}(\tilde{n}\pi k – \pi))+\mathcal{O}(\tilde{n}).\square\]Conclusion: Supporting points \(kr := \tilde{n}\pi k\) of the smooth \(f(kr)\) yield for \(j\) like \(k\) interpolating in \(\mathcal{O}(\sigma n)\):\[{}^mf_n(t):=\tilde{n}(u^{\tilde{r}jt})^T(\delta_{jk}\underline{j}^m)(\tilde{u}^{jk})(\check{f}(kr)).\square\]Theorem (DFT fixed-point method): Every \(z \in {}^{\omega}\mathbb{C}\) of an arbitrary \(m\)-polynomial \(p(z) = 0\) with \(m \in [2, n] \cap \mathbb{N}\) for \(n := 2^r, r \in {}^{\nu}\mathbb{N}^*\) and coefficients from \({}^{\nu}\mathbb{C}\) can be (initiating) determined also in \(\mathcal{O}(\sigma n).\)

Proof and algorithm: Let \(U = (\tilde{u}^{jk})\) for \(j, k \in \mathbb{N}_{<n}, u :={\epsilon}^{\tilde{n} \hat{\underline{\pi}}}, q := \hat{z}\) and \(s_k := p(\tilde{2}u^k)\). A simple transform achieves \(|q| < \tilde{2}\) for all zeros \(\zeta\) of \(p(z)\) and \(p(0) = 1\). It follows from \(p(z) = \tilde{n}(q^j)^TUs = \tilde{n}\mu^Ts = 0\) the simplified iteration \(\mu^* = U_{1}^{-T}\mu U((\delta_{jk}\tilde{u}^j)U^{-1}\mu-(U_{\acute{n}}^{-T}\mu+\beta s^T\mu, \beta s^T\mu, …, \beta s^T\mu)^T)\). Here let \(\delta_{jk}\) the Kronecker delta, starting point \(q := \tilde{2}\) and \(\beta \in {}^{\nu}\mathbb{C}^*\) such that each time \(||\mu^*-\mu||\) is roughly halved and \(\mu^Ts = 0\) holds. Finish by polynomial division where \(m > 2.\square\)

Remark: The identity instead of \(\delta_n^*\) provides arbitrarily precise approximations for the \({}^nf\). The last theorems are equally valid for multidimensional TS and Laurent series. The error is \(\mathcal{O}(\varepsilon^n)\) instead of \(\mathcal{O}(\varepsilon)\) for analogously defined \(m\)-dimensional DFT forms of the TS with \(\binom{m+n}{n}\) derivatives where the effort is comparable. A good choice for the crown points \(\varepsilon u^m + a\) around the crown centre \(a\) is \(\varepsilon = \tilde{2}\) and \(n = 64\).

Approximation theorem: The \({}^sf(x) \in {}^{\omega}\mathbb{R}\) for \(x \in {}^{\omega}\mathbb{R}\) as above allow computing the interpolant

\(g(x) := {\LARGE{\textbf{+}}}_{r=0}^{\acute{m}}{\chi_{]x_r, x_{\grave{r}}[}(x)((x_{\grave{r}}-x)p_r(x)+(x-x_r)p_{\grave{r}}(x))/(x_{\grave{r}}-x_r)}+\)\({\LARGE{\textbf{+}}}_{r=0}^m{\chi_{\{x_r\}}(x)p_r(x)}\)

for \(m, n \in {}^{\nu}\mathbb{N}\) and \(p_r(x) := {\LARGE{\textbf{+}}}_{s=0}^n{{}^sf(x_r){(x-x_r)}^s/s!}\) in \(\mathcal{O}(\sigma mn)\) where \({}^sf(x_r) = {}^sg(x_r)\) holds for every \(x_r \in {}^{\omega}\mathbb{R}\). Replace in the complex case \({}^{\omega}\mathbb{R}\) by \({}^{\omega}\mathbb{C}\) and put \(x = \gamma(t) \in {}^{\omega}\mathbb{C}\) for the path \(\gamma(t)\) where \(t \in {}^{\omega}\mathbb{R}.\square\)

Theorem (differential equations): Let \(d(z) \in {}^{\omega}\mathbb{C}^{n+2}\) vector of DFT forms for \(n \in {}^{\omega}\mathbb{N}\). If the system \({}^{\grave{n}}y = \left({}^ny,\ {}^{\acute{n}}y, …, y, 1\right)d(z) \in {}^{\omega}\mathbb{C}\) where (not) \(\left({}^ny(a),\ {}^{\acute{n}}y(a), …, y(a)\right)^T\in {}^{\omega}\mathbb{C}^{\grave{n}}\) and \(a, z \in {}^{\omega}\mathbb{C}\) has the solution \(y_0\), this can be determined in \(\mathcal{O}(n^3)\) giving the error mentioned above.\(\square\)

Remark: If function values of crown points are evaluated and terminating TS or polynomials are used, both a polynomial division may be executed and non-linear or partial (systems of) differential equations may be solved via DFT forms.

Example: For \(b = (f(\varepsilon u^0 + a), …, f(\varepsilon u^{\acute{n}} + a), 1)^T \in {}^{\omega}\mathbb{C}^{\grave{n}}\) and an upper triangle matrix \(T(z) \in {}^{\omega}\mathbb{C}^{\grave{n}\times\grave{n}}\), ordinary differential equations have only solutions if \(T(z)b = 0\) holds (Cholesky decomposition!).

Definition: To increase precision, let according to the trapezoidal rule \({}^{T}{\uparrow}_{z\in A}{f(z){\downarrow}z:={\LARGE{\textbf{+}}}_{z\in A}{\tilde{2}(f(z)+f(\overset{\rightharpoonup}{z}))(\overset{\rightharpoonup}{z}-z)}}\) and according to the midpoint rule \({}^{M}{\uparrow}_{z\in A}{f(z){\downarrow}z:={\LARGE{\textbf{+}}}_{z\in A}{f(\tilde{2}(z\,+\overset{\rightharpoonup}{z}}))(\overset{\rightharpoonup}{z}-z)}.\triangle\)

Series theorem for integrals: For \(c \in [0, a], f \in {}^{\nu}\mathcal{C}^{n}\) and \(n, \hat{p}, r\,(= 2^p), t\,(=2^v), \hat{v} \in {}^{\nu}2\mathbb{N}^*\), DFT forms of TS show

\({\uparrow}_0^a f(w){\downarrow}w = {\uparrow}_0^{\tilde{t}a}{\LARGE{\textbf{+}}}_{s=1}^t{f(x+\acute{s}\tilde{t}a){\downarrow}x} = {\uparrow}_0^1 g(y){\downarrow}y\)\(= {\LARGE{\textbf{+}}}_{\check{q}=1}^{\check{r}}{{\LARGE{\textbf{+}}}_{\check{m}=0}^{\check{n}-1}{\widetilde{\grave{m}!}\ {}^m\hat{g}\left (\tilde{r}\acute{q} \right){\tilde{r}}^{\grave{m}}}}+\mathcal{O}\left(\widetilde{\grave{n}!}\ {}^ng(\tilde{a}c){\tilde{r}}^n\right).\square\)

Remarks: DFT equivalents may replace any \({}^mg\) for \(g(y) := af(ay)\) and \(a > 0\). Transforming \(z := {\epsilon}^{\pm x}\) makes here infinite bounds of integration finite. A finite integral decreases the remainder’s modulus sufficiently. The midpoint rule is here more advantageous than the trapezoidal rule.

Example: For \({}_{\mp}g(\vartheta) := \check{\pi}(1 – {\varepsilon}^2 {\sin}^2(\check{\pi}\vartheta))^{\mp\tilde{2}}\), complete elliptic integrals of the first and second kind are \[{\uparrow}_0^{1} {}_{\mp}g(\vartheta) {\downarrow}\vartheta = {}_{\mp}g(\check{1}) + \widetilde{24}\ {}_{\mp}^2g(\check{1}) + \widetilde{1920}\ {}_{\mp}^4g(\check{1}) + \mathcal{O}\left(\widetilde{9!}\ {}_{\mp}^6g(\check{1})\right).\]Second Euler-Maclaurin formula: For \(f(\check{q}) = g(\tilde{r}\acute{q})\) and \(k = \grave{a} = \grave{r}\), the preceding theorem yields in \(\mathcal{O}(\sigma n)\)\[{\LARGE{\textbf{+}}}_{\check{q}=1}^{\check{r}} f(\check{q}) = {\uparrow}_{\check{1}}^{\check{k}} f(x){\downarrow}x + {\LARGE{\textbf{+}}}_{\check{m}=1}^{\check{n}-1} H_m \left ({}^{\acute{m}}f(\check{k}) – {}^{\acute{m}}f(\check{1}) \right ) + \mathcal{O}\left (H_n \left ({}^{\acute{n}}f(\check{k}) – {}^{\acute{n}}f(\check{1}) \right )\right ).\]Proof: For \(h(x)=x/\)sin \(x\) and \(H_m := \underline{1}^m\widetilde{m!}\ {}^mh(0)=\widetilde{m!}B_m(2-2^m) \rightarrow 2\tilde{\underline{\pi}}\text{-}^m\),\[{\LARGE{\textbf{+}}}_{\check{q}=1}^{\check{r}} g(\tilde{r}\acute{q}) = \check{r}{\uparrow}_0^1 g(y){\downarrow}y – {\LARGE{\textbf{+}}}_{\check{q}=1}^{\check{r}}{\LARGE{\textbf{+}}}_{\check{m}=1}^{\check{n}-1} {\widetilde{\grave{m}!}\ {}^mg(\tilde{r}\acute{q})} + \mathcal{O}\left(\widetilde{\grave{n}!}\ {}^ng(\tilde{a}c){\tilde{r}}^n\right)\]results in the claim by inserting and combining the factorials that are counted depending on partitions where \(B_m\) are Bernoulli numbers and \(B_{\grave{m}} = 0, B_0=1\) as well as \(B_1=-\tilde{2}.\square\)

First Euler-Maclaurin formula: Mathematical induction for \(n\) shows also that⁸cf. Mollin, Richard A.: Advanced Number Theory with Applications; 1st Ed.; 2017; Routledge; Milton Park, p. 193 f.

\({\LARGE{\textbf{+}}}_{\check{q}=0}^{\check{r}}f(\check{q})=\uparrow_0^{\check{r}}{f\left(x\right){\downarrow}x}+\check{f}(\check{r})+\check{f}(0)+{\LARGE{\textbf{+}}}_{\check{m}=1}^{\check{n}-1}{\widetilde{m!}}B_m\left({}^{\acute{m}}f(\check{r})-{}^{\acute{m}}f(0)\right)+\mathcal{O}\left({\widetilde{n!}}B_n\left({}^{\acute{n}}f(\check{r})-{}^{\acute{n}}f(0)\right)\right).\square\)

code of the FFT form

© 2010-2024 by Boris Haase

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