Adrian Hernandez Vega (2026). Fractional Gamma Iterations: Extending the Vigneras Hierarchy. Retrieved from http://adrianhv.com/projects/fractionalvigneras/
Definitions #
Definition 1 (Geometric Function). A geometric function is formed whenever an initial value is continuously multiplied by a constant factor r.
$$ a_0 \xrightarrow{\times r} a_0r \xrightarrow{\times r} a_0r^2 \xrightarrow{\times r} a_0r^3 \xrightarrow{\times r} \cdots $$Definition 2 (Hypergeometric Function). Whenever a function’s term ratio depends on a rational function \(R(n)\), the function becomes hypergeometric. Geometric functions are a subcase of hypergeometric functions.
$$ a_0 \xrightarrow{\times R(0)} a_1 \xrightarrow{\times R(1)} a_2 \xrightarrow{\times R(2)} a_3 \xrightarrow{\times R(3)} \cdots $$Since every term can be expressed as a product of consecutive ratios, the general term can be expressed as:
$$ \boxed{a_{n}=a_{0}\prod_{j=0}^{n-1}R\left(j\right)} $$Definition 3 (q-Series). A “\(q\)-series” is defined as a series with summands of the form \((1-a)(1-aq) \dots(1-aq^{n-1})\), which often involves the products of the form \((1-aq^k)\).
Definition 4 (\(q\)-Deformation). The \(q\)-deformation of an integer is defined as:
$$ [n]_q = \frac{1-q^n}{1-q} $$where \([n]_1\) is equal to \(n\) by \(\lim_{q \to 1} \frac{1-q^n}{1-q} = n\). Given a function’s expansion, a \(q\)-deformation can be formed by replacing the factorial term with the \(q\)-factorial, which is formed by the product of preceding \([n]_q\).
Definition 5 (Big Theta \(\Theta\)). A function is said to be in \(\Theta(g(n)\) if and only if there exist positive constants such that:
$$ 0 \leq c_1 g(n) \leq f(n) \leq c_2 g(n) $$Definition 6 (Logarithmic Profile). Let the Logarithmic Profile (Log-Profile) be the complexity class of \(f(x)\) under \(\Theta (\ln f(x))\).
Formal Classification #
To formally categorize the members of our hierarchy beyond Ackermann growth observations, we can calculate the Log-Profile for each member, resulting in distinct complexity orders.
Theorem 8 (Log-Profile of the Geometric Function). The Log-Profile of the Geometric Function has complexity \(\Theta(n)\).
Proof. Take an arbitrary geometric function: \(a\cdot r^{n-1}\), where \(a\) and \(r\) are constants.
$$ \begin{aligned} \ln (a\cdot r^{n-1}) &= \ln a+\left(n-1\right)\ln r \\ & \sim n\ln r \\ \end{aligned} $$Since \(\ln r\) is a mere constant, then the resulting complexity is simply \(\Theta(n)\).
Theorem 9 (Log-Profile of Gaussian Hypergeometric). The Log-Profile of the Gaussian Hypergeometric has complexity \(\Theta(\ln n)\).
Proof. Given a Gaussian hypergeometric series, the general coefficient is
$$ A_n=\frac{(a)_n (b)_n}{(c)_n n!} = \frac{\prod_{k=0}^{n-1} (a+k) \prod_{k=0}^{n-1} (b+k)}{\prod_{k=0}^{n-1} (c+k) \cdot n!} $$Applying the logarithmic transformation:
$$ \begin{aligned} \ln A_n &= \ln \left( \frac{\prod_{k=0}^{n-1} (a+k) \prod_{k=0}^{n-1} (b+k)}{\prod_{k=0}^{n-1} (c+k) \cdot n!} \right) \\ & = \ln \left( \prod_{k=0}^{n-1} (a+k) \right) + \ln \left( \prod_{k=0}^{n-1} (b+k) \right) -\ln \left( \prod_{k=0}^{n-1} (c+k) \right) - \ln n! \end{aligned} $$Turning products into Gamma functions:
$$ \begin{aligned} & \ln \left( \prod_{k=0}^{n-1} (a+k) \right) + \ln \left( \prod_{k=0}^{n-1} (b+k) \right) -\ln \left( \prod_{k=0}^{n-1} (c+k) \right) - \ln n! \\ & = \ln\frac{\Gamma(a+n)}{\Gamma(a)} + \ln\frac{\Gamma(b+n)}{\Gamma(b)} - \ln\frac{\Gamma(c+n)}{\Gamma(c)} -\ln n!\\ & \sim \left(a+n\right)\ln n-\left(a+n\right) + \left(b+n\right)\ln n-\left(b+n\right) - \left(\left(c+n\right)\ln n-\left(c+n\right) \right) -n\ln n \\ & \sim \left(\ln n-1\right)\left(a+b-c+n\right) -n\ln n \\ & \implies \ln(A_n) \in \Theta(\ln n) \end{aligned} $$
Theorem 10 (Log-Profile of q-Series). The Log-Profile of the q-series of the third level have complexity \(\Theta(n^2)\).
Proof. Given a q-series, the general coefficient is
$$ (a;q)_k=\prod_{k=0}^{n-1} (1 - a q^k). $$Applying the logarithm transforms the product into a sum:
$$ \ln \left| \prod_{k=0}^{n-1} (1 - a q^k) \right| = \sum_{k=0}^{n-1} \ln |1 - a q^k|. $$For \(|q| > 1\), the term \(|aq^k|\) dominates \(1\) for large \(k\), so asymptotically:
$$ \sum_{k=0}^{n-1} \ln|1 - aq^k| \sim \sum_{k=0}^{n-1} \ln |a q^k| = \sum_{k=0}^{n-1} (\ln|a| + k \ln|q|) = n \ln|a| + \frac{n(n-1)}{2} \ln|q| = \Theta(n^2). $$For \(|q| < 1\), the quadratic term is negative:
$$ \sum_{k=0}^{n-1} \ln|a q^k| = n \ln|a| + \frac{n(n-1)}{2} \ln|q| = -\Theta(n^2), $$but the magnitude still grows like \(n^2\), so asymptotically we have \(\Theta(n^2)\) in all cases.
The Vigneras Extension #
Theorem 11 (Log-Profile of the Vigneras Multi-Gamma Functions). The Log-Profile of the Vigneras \(G_n(x)\) function has complexity \(\Theta(x^n \ln x)\).
Proof. We proceed by way of mathematical induction, showing the base case (\(n=1\)) and then proving for general \(n\) with \(n-1\) as inductive hypothesis.
Base Step: \(n=1\)
Gamma is defined to be the first case of the Vigneras hierarchy; using Stirling’s approximation and \(\Gamma(x) = (x-1)!\),
$$ \Gamma(x) \sim \sqrt{2\pi} (x-1)^{x-\frac{1}{2}} e^{-(x-1)}. $$Taking logarithms,
$$ \ln \Gamma(x) \sim \ln(\sqrt{2\pi}) + \left(x-\tfrac{1}{2}\right)\ln(x-1) - (x-1). $$
Since \(\ln(x-1) = \ln x + o(1)\), it follows that
$$ \begin{aligned} \ln \Gamma(x) &\sim x\ln x - x. \\ & \implies \ln G_1(x) \in \Theta(x^1\ln x) \end{aligned} $$
Inductive Step: \(n-1\)
Assume as inductive hypothesis that the Log-Profile of \(G_{n-1}(x)\) has complexity \(\Theta(x^n \ln x)\).
$$ G_n\left(x+1\right)=\prod_{k=0}^{x-1} G_{n-1}(k)=G_{n-1}(0)\cdot G_{n-1}(1)\cdot G_{n-1}(2) ...G_{n-1} (x-1) $$
A level \(G_n\) can be written as a finite product of the previous level.Taking the logarithm allows us to convert the right into summations:
$$ \begin{aligned} & \ln\left(G_{n}\left(x+1\right)\right)=\sum_{k=1}^{x-1}\ln\left(G_{n-1}(k)\right) \end{aligned} $$Substitute the summand with the inductive hypothesis and rewrite in terms of an integral:
$$ \begin{aligned} \ln\left(G_{n}\left(x+1\right)\right)& =\sum_{k=1}^{x-1}\ln\left(G_{n-1}\left(k\right)\right) \\ & \sim\sum_{k=1}^{x-1}k^{n-1}\ln k \\ & = \int_{1}^{x-1}\left(k^{n-1}\ln k\right)\text{dk}+\text{o}\left(k^{n-1}\ln k\right)\\ \end{aligned} $$With Integration by Parts, the resulting integral can be written as:
$$ \begin{aligned} &\frac{k^{n}}{n}\ln k-\int_{ }^{ }\left(\frac{1}{k}\right)\left(k^{n}\right)\text{dk}+\text{o}\left(k^{n-1}\ln k\right) \\ & = \boxed{\frac{k^{n}}{n}\ln k-\frac{k^{n}}{n+1} + \text{o}\left(k^{n-1}\ln k\right)} \\ \end{aligned} $$Since the only remaining term is \(\text{o}\left(k^{n-1}\ln k\right)\), which must grow slower than \(k^n\ln k\), the complexity of \(G_n\) must be \(\Theta (x^n \ln x)\)
Theorem 12. Log-Profile of the Fractional Vigneras Multi-Gamma Functions The Log-Profile of the Fractional Vigneras \(G_\alpha(x)\) function has complexity \(\Theta(x^\alpha \ln x)\).
Proof. Since induction only holds for discrete cases as shown above, we must generalize the concept of multiple integration to fractional applications. To define the continuous analog of the Vignéras hierarchy, we utilize the Riemann-Liouville fractional integral operator \(J^\alpha\) of order \(\alpha\). For a base function \(f(t) = \ln t\), the operator is defined as:
$$ J^\alpha (\ln x) = \frac{1}{\Gamma(\alpha)} \int_{0}^{x} (x - t)^{\alpha - 1} \ln(t) \, dt $$To evaluate the asymptotic growth of this integral, we employ the same integration by parts strategy. Integrating the power function and differentiating the logarithm yields the exact analytical closed-form solution:
$$ J^\alpha (\ln x) = \frac{x^\alpha}{\Gamma(\alpha + 1)} \left[ \ln(x) - \psi(\alpha + 1) - \gamma \right] $$where \(\psi(z)\) is the digamma function and \(\gamma\) is the Euler-Mascheroni constant.
For a fixed fractional order \(\alpha\), both \(\psi(\alpha + 1)\) and \(\gamma\) are finite constants. As \(x \to \infty\), the term \(\ln(x)\) strictly dominates the bracketed expression. Thus, the leading term of the expansion reduces to:
$$ J^\alpha (\ln x) \sim \frac{1}{\Gamma(\alpha + 1)} x^\alpha \ln(x) $$This completes the proof that \(\ln G_\alpha(x)\) scales at a rate of \(\Theta(x^\alpha \ln x)\). Furthermore, for integer values \(\alpha = n\), the coefficient cleanly collapses to the expected discrete bound of \(x^n \ln x\), matching the previous theorem, as well as known literature. ∎
Theorem 13 (Vigneras Extension of the Bohr-Mollerup Theorem). Each function in the Vigneras hierarchy is uniquely characterized by the following three requirements:
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Functional Equation: \(G_n (x+1) = G_{n-1}(x) G_n(x) \forall x>0\)
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Normalization: \(G_n(1) = 1, \forall n \geq 1\)
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Positive Derivative: Non negative \((n+1)\)th derivative.
Corollary (Vigneras Extension of the Bohr-Mollerup Theorem). Fractional members of the Vigneras hierarchy satisfy the three Bohr-Mollerup criteria, but are not unique. Positive derivative follows from the previous derivation, and normalization can be achieved with \(\tilde{f}(x) = \frac{f(x)}{f(1)}\).
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