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In mathematics, the Fabius function is an example of an infinitely differentiable function that is nowhere analytic, found by Jaap Fabius (1966).

This function satisfies the initial condition f ( 0 ) = 0 {\displaystyle f(0)=0} , the symmetry condition f ( 1 − − x ) = 1 − − f ( x ) {\displaystyle f(1-x)=1-f(x)} for 0 ≤ ≤ x ≤ ≤ 1 , {\displaystyle 0\leq x\leq 1,} and the functional differential equation

f ′ ( x ) = 2 f ( 2 x ) {\displaystyle f'(x)=2f(2x)}

for 0 ≤ ≤ x ≤ ≤ 1 / 2. {\displaystyle 0\leq x\leq 1/2.} It follows that f ( x ) {\displaystyle f(x)} is monotone increasing for 0 ≤ ≤ x ≤ ≤ 1 , {\displaystyle 0\leq x\leq 1,} with f ( 1 / 2 ) = 1 / 2 {\displaystyle f(1/2)=1/2} and f ( 1 ) = 1 {\displaystyle f(1)=1} and f ′ ( 1 − − x ) = f ′ ( x ) {\displaystyle f'(1-x)=f'(x)} and f ′ ( x ) + f ′ ( 1 2 − − x ) = 2. {\displaystyle f'(x)+f'({\tfrac {1}{2}}-x)=2.}

It was also written down as the Fourier transform of

f ^ ^ ( z ) = ∏ ∏ m = 1 ∞ ∞ ( cos ⁡ ⁡ π π z 2 m ) m {\displaystyle {\hat {f}}(z)=\prod _{m=1}^{\infty }\left(\cos {\frac {\pi z}{2^{m}}}\right)^{m}}

by Børge Jessen and Aurel Wintner (1935).

The Fabius function is defined on the unit interval, and is given by the cumulative distribution function of

∑ ∑ n = 1 ∞ ∞ 2 − − n ξ ξ n , {\displaystyle \sum _{n=1}^{\infty }2^{-n}\xi _{n},}

where the ξn are independent uniformly distributed random variables on the unit interval. That distribution has an expectation of 1 2 {\displaystyle {\tfrac {1}{2}}} and a variance of 1 36 {\displaystyle {\tfrac {1}{36}}} .

There is a unique extension of f to the real numbers that satisfies the same differential equation for all x. This extension can be defined by f (x) = 0 for x ≤ 0, f (x + 1) = 1 − f (x) for 0 ≤ x ≤ 1, and f (x + 2r) = −f (x) for 0 ≤ x ≤ 2r with r a positive integer. The sequence of intervals within which this function is positive or negative follows the same pattern as the Thue–Morse sequence.

The Rvachëv up functioncite-ref-1[1] is closely related: u ( t ) = { F ( t + 1 ) , | t | < 1 0 , | t | ≥ ≥ 1 {\displaystyle u(t)={\begin{cases}F(t+1),\quad |t|<1\\0,\quad |t|\geq 1\end{cases}}} which fulfills the Delay differential equationcite-ref-2[2] d d t u ( t ) = 2 u ( 2 t + 1 ) − − 2 u ( 2 t − − 1 ) . {\displaystyle {\frac {d}{dt}}u(t)=2u(2t+1)-2u(2t-1).} (see Another example).

• Values

• Asymptotic

• References

The Fabius function is constant zero for all non-positive arguments, and assumes rational values at positive dyadic rational arguments. For example:cite-ref-3[3]cite-ref-4[4]

• f ( 1 ) = 1 {\displaystyle f(1)=1}

• f ( 1 2 ) = 1 2 {\displaystyle f({\tfrac {1}{2}})={\tfrac {1}{2}}}

• f ( 1 4 ) = 5 72 {\displaystyle f({\tfrac {1}{4}})={\tfrac {5}{72}}}

• f ( 1 8 ) = 1 288 {\displaystyle f({\tfrac {1}{8}})={\tfrac {1}{288}}}

• f ( 1 16 ) = 143 2073600 {\displaystyle f({\tfrac {1}{16}})={\tfrac {143}{2073600}}}

• f ( 1 32 ) = 19 33177600 {\displaystyle f({\tfrac {1}{32}})={\tfrac {19}{33177600}}}

• f ( 1 64 ) = 1153 561842749440 {\displaystyle f({\tfrac {1}{64}})={\tfrac {1153}{561842749440}}}

• f ( 1 128 ) = 583 179789679820800 {\displaystyle f({\tfrac {1}{128}})={\tfrac {583}{179789679820800}}}

with the numerators listed in OEIS: A272755 and denominators in OEIS: A272757.

log ⁡ ⁡ f ( x ) = − − log 2 ⁡ ⁡ x 2 log ⁡ ⁡ 2 + log ⁡ ⁡ x ⋅ ⋅ log ⁡ ⁡ ( − − log ⁡ ⁡ x ) log ⁡ ⁡ 2 − − ( 1 2 + 1 + log ⁡ ⁡ log ⁡ ⁡ 2 log ⁡ ⁡ 2 ) log ⁡ ⁡ x − − log 2 ⁡ ⁡ ( − − log ⁡ ⁡ x ) 2 log ⁡ ⁡ 2 + log ⁡ ⁡ log ⁡ ⁡ 2 ⋅ ⋅ log ⁡ ⁡ ( − − log ⁡ ⁡ x ) log ⁡ ⁡ 2 + ( 6 γ γ 2 + 12 γ γ 1 − − π π 2 − − 6 log 2 ⁡ ⁡ log ⁡ ⁡ 2 12 log ⁡ ⁡ 2 − − 7 log ⁡ ⁡ 2 12 − − log ⁡ ⁡ π π 2 ) + log 2 ⁡ ⁡ ( − − log ⁡ ⁡ x ) 2 log ⁡ ⁡ 2 ⋅ ⋅ log ⁡ ⁡ x − − log ⁡ ⁡ log ⁡ ⁡ 2 ⋅ ⋅ log ⁡ ⁡ ( − − log ⁡ ⁡ x ) log ⁡ ⁡ 2 ⋅ ⋅ log ⁡ ⁡ x + O ( 1 log ⁡ ⁡ x ) {\displaystyle {\begin{aligned}\log f(x)&=-{\frac {\log ^{2}x}{2\log 2}}+{\frac {\log x\cdot \log(-\log x)}{\log 2}}-\left({\frac {1}{2}}+{\frac {1+\log \log 2}{\log 2}}\right)\log x-{\frac {\log ^{2}(-\log x)}{2\log 2}}+{\frac {\log \log 2\cdot \log(-\log x)}{\log 2}}\\&+\left({\frac {6\gamma ^{2}+12\gamma _{1}-\pi ^{2}-6\log ^{2}\log 2}{12\log 2}}-{\frac {7\log 2}{12}}-{\frac {\log \pi }{2}}\right)+{\frac {\log ^{2}(-\log x)}{2\log 2\cdot \log x}}-{\frac {\log \log 2\cdot \log(-\log x)}{\log 2\cdot \log x}}+O\!\left({\frac {1}{\log x}}\right)\end{aligned}}}

for x → → 0 + , {\displaystyle x\to 0^{+},} where γ γ {\displaystyle \gamma } is Euler's constant, and γ γ 1 {\displaystyle \gamma _{1}} is the Stieltjes constant. Equivalently,

log ⁡ ⁡ f ( 2 − − n ) = − − n 2 log ⁡ ⁡ 2 2 − − n log ⁡ ⁡ n + ( 1 + log ⁡ ⁡ 2 2 ) n − − log 2 ⁡ ⁡ n 2 log ⁡ ⁡ 2 + ( 6 γ γ 2 + 12 γ γ 1 − − π π 2 12 log ⁡ ⁡ 2 − − 7 log ⁡ ⁡ 2 12 − − log ⁡ ⁡ π π 2 ) − − log 2 ⁡ ⁡ n 2 n log 2 ⁡ ⁡ 2 + O ( 1 n ) {\displaystyle \log f\!\left(2^{-n}\right)=-{\frac {n^{2}\log 2}{2}}-n\log n+\left(1+{\frac {\log 2}{2}}\right)n-{\frac {\log ^{2}n}{2\log 2}}+\left({\frac {6\gamma ^{2}+12\gamma _{1}-\pi ^{2}}{12\log 2}}-{\frac {7\log 2}{12}}-{\frac {\log \pi }{2}}\right)-{\frac {\log ^{2}n}{2n\log ^{2}2}}+O\!\left({\frac {1}{n}}\right)}

for n → → ∞ ∞ . {\displaystyle n\to \infty .}

cite-note-11. ↑ "A288163 - Oeis".

cite-note-22. ↑ citerefjuan-arias-de-reyna2017Juan Arias de Reyna (2017). "Arithmetic of the Fabius function". arXiv:1702.06487 [math.NT].

cite-note-33. ↑ citerefsloane-a272755Sloane, N. J. A. (ed.). "Sequence A272755 (Numerators of the Fabius function F(1/2^n).)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.

cite-note-44. ↑ citerefsloane-a272757Sloane, N. J. A. (ed.). "Sequence A272757 (Denominators of the Fabius function F(1/2^n).)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.

• citereffabius1966Fabius, J. (1966), "A probabilistic example of a nowhere analytic C ∞-function", Zeitschrift für Wahrscheinlichkeitstheorie und Verwandte Gebiete, 5 (2): 173–174, doi:10.1007/bf00536652, MR 0197656, S2CID 122126180

• citerefjessenwintner1935Jessen, Børge; Wintner, Aurel (1935), "Distribution functions and the Riemann zeta function", Trans. Amer. Math. Soc., 38: 48–88, doi:10.1090/S0002-9947-1935-1501802-5, MR 1501802

• citerefdimitrov2006Dimitrov, Youri (2006). Polynomially-divided solutions of bipartite self-differential functional equations (Thesis).

• citerefarias-de-reyna2017Arias de Reyna, Juan (2017). "Arithmetic of the Fabius function". arXiv:1702.06487 [math.NT].

• citerefarias-de-reyna2017Arias de Reyna, Juan (2017). "An infinitely differentiable function with compact support: Definition and properties". arXiv:1702.05442 [math.CA]. (an English translation of the author's paper published in Spanish in 1982)

• Alkauskas, Giedrius (2001), "Dirichlet series associated with Thue-Morse sequence", preprint.

• Rvachev, V. L., Rvachev, V. A., "Non-classical methods of the approximation theory in boundary value problems", Naukova Dumka, Kiev (1979) (in Russian).