A Class of Differential Operators and the Stirling Numbers

The differential operator \displaystyle (x^{1+y} \; D)^n with \displaystyle D=d/dx can easily be expanded in terms of the operators \displaystyle (:xD:)^n = x^n \; D^n by considering its action on \displaystyle x^s \; .

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Fractional calculus and interpolation of generalized binomial coefficients


Interpolation of the generalized binomial coefficients underlie the representation of a particular class of fractional differintegro operators by convolution integrals and Cauchy-like complex contour integrals.

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Newton Interpolation and the Derivative in Finite Differences

Relations between the normalized Mellin transform (MT) and Newton interpolation (NI) can shed some light on the validity of a finite difference formula for the derivative alluded to in the MathOverflow question MO-Q: Derivative in terms of finite differences.

From formal symbolic calculus, the forward finite difference is defined by

\displaystyle \triangle_x f(x) = f(x+1) - f(x) = (e^{D_x}-1)f(x) \;,

so inverting gives

\displaystyle D_x = \log(1 + \triangle_x) = \sum_{n=1}^{\infty} (-1)^{n+1} \frac{\triangle_x^n}{n}.

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Representing integration in the reciprocal spaces of the Fourier and Laplace transforms

There’s some confusion concerning representations of integration in the reciprocal spaces of the Fourier and Laplace transforms in some entries of MathOverflow that arises from not distinguishing among integration operators with different limits of integration (combined with some handwaving about group characters). Misassociation, or conflation, of distinct integration ops has been a source of historical confusion (cf. [Threefold Interpretation of Fractional Derivatives][1] by R. Hilfer as well as the comments in [MO question][2]). The convolution theorems provide a way to view differing integrations to effectively translate them into relatively simple factors in the reciprocal spaces.

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The Riemann and Hurwitz zeta functions and the Mellin transform interpolation of the Bernoulli polynomials

This entry (expanding on the Bernoulli Appells entry) illustrates interpolation with the Mellin transform of the Bernoulli polynomials and their umbral inverses, the reciprocal polynomials, giving essentially the Hurwitz zeta function and the finite difference of x^{1-s}/(1-s), both of which can be umbrally inverted by the polynomials. It also elaborates on a set of generalized Bernoulli polynomials based on umbral composition of the Bernoulli polynomials with themselves and derives an “asymptotic” expression, or divergent series, for the Riemann zeta function noted in the Bernoulli Appells entry, which may be truncated to give very good approximations of the Hurwitz and Riemann zeta functions over ranges of parameters.

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Appell polynomials, cumulants, noncrossing partitions, Dyck lattice paths, and inversion

The raising op for any Appell sequence is determined by the derivative of the log of the e.g.f. of the basic number sequence, connecting the op to the combinatorics of the cumulant expansion OEIS-127671 of the moment generating function and its inverse relation A036040 for the general Bell polynomials of the Faa di Bruno formula for composition of functions. Diagrammatics of the partitions for the combinatorics of these entries can be found in the statistical physics references of A036040, but are by no means unique.

Furthermore, the combinatorics of the classical cumulants are, at a combinatorial level, intimately allied to that of the free cumulants of free probability theory and, consequently, to noncrossing partitions, as discussed by Jonathan Novak in “Three lectures on free probability”, Roland Speicher in “Free probability theory and non-crossing partitions”, Franz Lehner and coauthors in papers noted in the Bernoulli Appells entry, and by Ardila, Rincon, and Williams in “Positroids and noncrossing partitions”.

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The Hirzebruch criterion for the Todd class

The Hirzebruch criterion for the Todd class is given in “The signature theorem: reminiscences and recreations” by Hirzebruch. The formal power series s(t) that defines the Todd class must satisfy \frac{d^n}{dt^n} (s(t))^{n+1} |_{t=0} = n! . The e.g.f. for the Bernoulli numbers uniquely satisfies this criterion. I’d like to make a note of how the Bernoullis and the integer reciprocals are really two sides of the same coin and how both play a role in the Todd class criterion, and then note the relation to some important combinatorics, through a Lagrange inversion formula (LIF).

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