Bessel polynomials are a class of orthogonal polynomials of degree n, defined explicitly as y_n(x) = \sum_{k=0}^n \frac{(n+k)!}{k!(n-k)!} \left( \frac{x}{2} \right)^k, which satisfy the second-order differential equation x^2 y'' + (2x + 2) y' - n(n + 1) y = 0.[1] Introduced by H. L. Krall and Orrin Frink in 1949, they form a fourth family of classical orthogonal polynomials alongside the Jacobi, Laguerre, and Hermite polynomials, distinguished by their orthogonality on the unit circle with respect to the weight function e^{-2/x}.[1] These polynomials arise naturally in the solution of the classical wave equation in spherical polar coordinates, where they describe the radial component of traveling spherical waves.[1] Their name derives from a close relation to the modified Bessel functions of the second kind, or Macdonald functions K_\nu(z), particularly through generating functions and hypergeometric representations such as y_n(x) = {}_2F_0(-n, n+1; ; -x/2).[2]Key properties include a three-term recurrence relation y_{n+1}(x) = (2n + 1) x \, y_n(x) + y_{n-1}(x) and Rodrigues' formula y_n(x) = 2^{-n} x^{-2n} \frac{d^n}{dx^n} (x^{2n} e^{-2/x}), which facilitate their computation and analysis.[1] The generating function \sum_{n=1}^\infty y_{n-1}(x) \frac{t^n}{n!} = \exp\left( \frac{1 - \sqrt{1 - 2 x t}}{x} \right) further connects them to exponential functions.[1] In applications, the reversed Bessel polynomials \theta_n(x) = x^n y_n(1/x) are employed in the design of Bessel filters, analog linear filters that provide maximally flat group delay for optimal transient response in signal processing and electronics.[3] Additionally, Bessel polynomials appear in numerical methods for evaluating inverse Laplace transforms and in modeling phenomena such as isotropic turbulence fields.[4] Extensions, including generalized and q-analogues, have been developed to broaden their utility in fractional calculus and quantum mechanics.[4]
History and Motivation
Discovery and Early Development
The Bessel polynomials were formally introduced as a new class of orthogonal polynomials in 1949 by H. L. Krall and Orrin Frink in their seminal paper published in the Transactions of the American Mathematical Society.[5] Although the polynomials appeared in earlier works, such as those by S. Bochner in 1929, W. Hahn in 1935, and H. L. Krall in 1938, Krall and Frink's paper established them as a distinct class, naming them after their close connection to modified Bessel functions of the second kind.[5]The primary motivation for their introduction stemmed from efforts to solve the classical wave equation in spherical polar coordinates, where solutions involving modified Bessel functions naturally led to polynomial expansions.[5] Krall and Frink recognized that these polynomials provided a useful orthogonal basis for representing such solutions, facilitating analysis in mathematical physics problems related to wave propagation.[5]In the early 1950s, researchers began extending the foundational work of Krall and Frink, focusing on key properties to broaden their applicability. For instance, J. L. Burchnall explored the relationship between Bessel polynomials and Bessel functions in greater detail, establishing important connections that supported further theoretical developments.[6] During this decade, advancements included refinements to explicit representations and recurrence relations, solidifying the polynomials' role in orthogonal systems and generating function theory.[7]
Physical and Mathematical Origins
Bessel polynomials emerged in the context of solving the classical wave equation in spherical polar coordinates, providing a mathematical framework for addressing propagation phenomena in physics. They are particularly connected to spherical wave propagation problems, where solutions to the Helmholtz equation, \nabla^2 u + k^2 u = 0, require expansions that align with radial symmetries in these geometries. This association arises because the polynomials facilitate series representations that capture the oscillatory and decaying behaviors inherent in wave solutions, such as those encountered in acoustics and electromagnetism.[7]From a broader mathematical perspective, Bessel polynomials were driven by the need to extend orthogonal polynomial theory to non-standard weight functions defined on contours in the complex plane. Unlike classical orthogonal polynomials on the real line, they satisfy orthogonality with respect to the weight e^{-2/x} integrated over the unitcircle |x| = 1 in the complex plane, allowing for applications in complex analysis and contour integration techniques. This innovation addressed gaps in representing functions with circular symmetries or exponential weights in the complex domain.[5]
Definitions
Generating Function Definition
The Bessel polynomials y_n(x) are related to their exponential generating function through the shifted index\sum_{n=1}^\infty y_{n-1}(x) \frac{t^n}{n!} = \exp\left( \frac{1 - \sqrt{1 - 2xt}}{x} \right),with y_0(x) = 1. This form provides a closed expression that encodes the sequence of polynomials as coefficients in the Taylor series expansion around t = 0.[8]The derivation of this generating function stems from the explicit sum formula for the polynomials, y_n(x) = \sum_{k=0}^n \frac{(n+k)!}{(n-k)! k!} \left( \frac{x}{2} \right)^k, by substituting into the exponential series and summing the double series term by term, leveraging identities from modified Bessel functions to simplify the result to the given closed form. This process involves expanding the exponential and collecting like terms in powers of t, where the reciprocal-like structure in the argument arises from the binomial expansion of the square root term (1 - 2xt)^{1/2}, which generates the necessary factorial ratios in the coefficients. The resulting expression is unique as an exponentialgenerating function, as it incorporates the n! scaling that reflects the polynomial degrees and facilitates combinatorial interpretations.[1]Immediate consequences of this generating function include the ability to compute individual y_n(x) by repeated differentiation with respect to t and evaluating [at t](/page/AT&T) = 0, multiplied by n!, or by direct series extraction, which connects the polynomials to broader exponential generating series techniques in analysis and combinatorics. This form also highlights the close relationship to modified Bessel functions of the second kind, where y_n(x) = \sqrt{\frac{2}{\pi x}} \, e^{1/x} \, K_{n + 1/2}\left( \frac{1}{x} \right). The exponential nature distinguishes it by enabling applications in differential equations and asymptotic analysis.[8][1]
Explicit Sum Formula
The explicit sum formula for the Bessel polynomial y_n(x) of degree n is given byy_n(x) = \sum_{k=0}^n \frac{(n+k)!}{k!(n-k)!} \left( \frac{x}{2} \right)^k.This representation was introduced in the seminal work defining the polynomials as solutions to a specific Sturm-Liouville problem on the complex unit circle.[9]The polynomials are normalized such that y_n(0) = 1, with the constant term corresponding to the k=0 summand, and the leading coefficient (for the x^n term) equal to \frac{(2n)!}{2^n n!}, ensuring they are not monic but scaled to reflect their orthogonal properties under the weight e^{-2/x} on the unit circle.[9][2] The coefficients a_{n,k} = \frac{(n+k)!}{k!(n-k)!} in the expansion exhibit a combinatorial structure, interpretable as the number of ordered perfect matchings with k increasing edges in the complete graph K_{2n} relative to a fixed Hamiltonian path, providing a graph-theoretic counting perspective on the polynomials.[10]For illustration, the first few Bessel polynomials are:
y_0(x) = 1,
y_1(x) = 1 + x,
y_2(x) = 1 + 3x + 3x^2.
These examples demonstrate the rapid growth in coefficients, with y_2(x) already showing quadratic scaling that aligns with the factorial terms in the sum.[9]
Hypergeometric Representation
The Bessel polynomials admit a representation in terms of the generalized hypergeometric function _2F_0, given byy_n(x) = \, _2F_0\left(-n,\, n+1;\, -;\, -\frac{x}{2}\right).This form arises directly from matching the coefficients of the explicit series expansion of y_n(x) to the general term of the hypergeometric series.[2]Due to the upper parameter -n, the Pochhammer symbol (-n)_k vanishes for k > n, ensuring the infinite series terminates after exactly n+1 terms and produces a polynomial of degree n. This termination property underscores the role of Bessel polynomials within the broader class of hypergeometric polynomials, where negative integer parameters enforce finite support.[2]An equivalent expression employs the confluent hypergeometric function _1F_1, namelyy_n(x) = (n+1)_n \left( \frac{x}{2} \right)^n \, _1F_1\left( -n;\, -2n;\, \frac{2}{x} \right),where (n+1)_n = \frac{(2n)!}{n!}. The confluent hypergeometric function emerges as a limiting case of the Gauss hypergeometric function _2F_1 when the second upper parameter approaches infinity while rescaling the argument, providing a conceptual bridge to non-terminating special functions.[2]These hypergeometric representations offer significant advantages for asymptotic analysis, as the large-argument or large-parameter behaviors of _2F_0 and _1F_1 are well-characterized through saddle-point methods and Stirling approximations, enabling precise estimates of y_n(x) for extreme regimes. Additionally, they support the derivation of integral representations, such as those via the Barnes contour integral for _2F_0 or the Kummer integral for _1F_1, which prove invaluable in solving boundary-value problems and establishing orthogonality. The explicit sum \sum_{k=0}^n \frac{(n+k)!}{k!(n-k)!} \left( \frac{x}{2} \right)^k corresponds precisely to the truncated series of these functions. Beyond polynomials, the forms allow analytic continuation to non-integer n, facilitating generalizations in special function theory.[2][11]
Relation to Bessel Functions
The Bessel polynomials are named after their intimate connection to Bessel functions, arising as polynomial solutions in contexts where modified Bessel functions provide the transcendental counterparts.A key relation expresses the Bessel polynomial exactly in terms of the modified Bessel function of the second kind:y_n(x) = \sqrt{\frac{2}{\pi x}} \, e^{1/x} \, K_{n + 1/2}\left( \frac{1}{x} \right),where K_\nu(z) denotes the modified Bessel function of the second kind. This representation holds for x > 0 and highlights how the polynomials emerge from the analytic properties of these functions in the limit of small arguments adjusted appropriately.[8]This link extends to asymptotic expansions. For large order n with fixed argument z, the modified Bessel function of the first kind admits the approximationI_n(z) \sim \frac{(z/2)^n}{n!} \, y_n(1/z),capturing the leading behavior while higher-order terms in the expansion account for deviations; this form is particularly useful when $1/z is small, aligning the polynomial's degree with the dominant contributions in the series.[8]The historical origin of this connection traces to 1949, when H. L. Krall and O. Frink introduced the polynomials to solve the three-dimensional wave equation in spherical coordinates, where the radial component involves expansions in Bessel functions of half-integer order for wave propagation. The defining transformation formula isy_n\left( \frac{1}{i r} \right) = \sqrt{\frac{\pi}{2}} \, r^{-1/2} \, e^{i r} \left[ i^{-n} J_{n+1/2}(r) + i^n J_{-n-1/2}(r) \right],with J_\nu(z) the Bessel function of the first kind; analytic continuation to imaginary arguments yields relations to the modified forms I_n(z) and K_n(z), bridging wave solutions in oscillatory and exponential regimes.[1]These transformations underscore the polynomials as a limiting case of Bessel functions when the order is fixed and the argument is scaled, facilitating computations in domains where transcendental evaluations are replaced by algebraic ones; the hypergeometric representation offers an intermediary path to derive such links.
Properties
Recurrence Relations
Bessel polynomials satisfy a three-term recurrence relation that facilitates the recursive computation of successive members of the sequence. The standard form isy_{n+1}(x) = (2n + 1)x \, y_n(x) + y_{n-1}(x),for n \geq 1, with initial conditions y_0(x) = 1 and y_1(x) = x + 1.[1] This relation enables efficient numerical evaluation and highlights the linear structure inherent to the polynomials.The recurrence can be derived through contiguous function relations stemming from the polynomials' representation as hypergeometric functions y_n(x) = {}_2F_0(-n, n+1; ; -x/2).[2] As a class of orthogonal polynomials, Bessel polynomials adhere to the general three-term recurrence structure typical of families orthogonal with respect to a positive definite moment functional, with orthogonality on the unit circle.[2] This connection underscores their role within broader orthogonal polynomial theory, where the recurrence preserves key properties like the degree and leading coefficient relations.For computational purposes, the forward recurrence is stable and well-conditioned when evaluating the polynomials for real x \geq 0 and moderate to large n, as the terms grow appropriately without significant cancellation.[12] However, in regions near the zeros (which lie in the left half of the complex plane) or for small n with complex arguments, a backward recurrence—starting from high degrees and iterating downward—enhances numerical stability by minimizing the propagation of rounding errors.[12] This approach leverages the asymptotic dominance of the polynomials for large n.
Differential Equation
The Bessel polynomials y_n(x) satisfy the second-order linear differential equationx^2 y''(x) + (2x + 2) y'(x) - n(n + 1) y(x) = 0.[1][2]This equation arises in the context of solutions to wave equations in spherical coordinates, where the polynomials emerge as particular solutions with the boundary condition y(0) = 1. The equation has a regular singular point at x = 0 and an irregular singular point at x = \infty. At x = 0, the Frobenius method yields the indicial equation r(r + 1) = 0, with roots r = 0 and r = -1. The root r = 0 generates a power series solution that terminates after n terms due to the structure of the recurrence coefficients, producing the polynomial y_n(x); the other root leads to a second solution involving negative powers.[2]The self-adjoint (Sturm-Liouville) form of the differential equation, which facilitates analysis of boundary value problems and orthogonality implications, is\frac{d}{dx} \left[ x e^{2/x} y'(x) \right] - n(n+1) x^{-1} e^{2/x} y(x) = 0.[2]This form highlights the role of the equation in singular Sturm-Liouville theory, where the polynomial solutions form an orthogonal basis with respect to a suitable weight function over appropriate contours.[2]
Orthogonality Relations
Bessel polynomials y_n(z) exhibit orthogonality over the complex plane, specifically along the unit circle |z| = 1, with respect to a non-standard weight function that distinguishes them from classical orthogonal polynomials defined on real intervals. Unlike the latter, which typically involve real-line integrals with positive weights, the Bessel polynomials' orthogonality arises in a contour integral setting, reflecting their origins in solutions to differential equations with complex singularities. This complex-domain orthogonality facilitates applications in analytic continuation and residue-based computations.[1][2]The precise orthogonality relation is given by\frac{1}{2\pi i} \int_{|z|=1} y_m(z) y_n(z) e^{-2/z} \, dz = \delta_{mn} (-1)^{n+1} \frac{2}{2n+1},where the integral is taken in the positive sense around the unit circle, and \delta_{mn} is the Kronecker delta. This formula holds for non-negative integers m and n. A related weight function can be expressed incorporating the exponential factor linking to the generating function for the polynomials.[2]This orthogonality can be proved using contour integration techniques applied to the exponential generating function of the Bessel polynomials, given by \exp\left( \frac{1 - \sqrt{1 - 2zt}}{z} \right) = \sum_{k=1}^\infty y_{k-1}(z) \frac{t^k}{k!}. By considering the residue at the origin of the product of two such generating functions and extracting coefficients via Cauchy's integral formula, the integral representation follows from evaluating the residues of the Laurent series expansion around z = 0. Alternatively, the proof leverages the self-adjoint form of the differential equation satisfied by the polynomials, transforming the boundary value problem into a contour integral where orthogonality emerges from the vanishing of off-diagonal terms due to analytic properties inside the unit disk. These methods highlight the role of residues in establishing the relation, with the norm arising from the explicit evaluation at equal degrees.[1][2]The consequences of this orthogonality include the ability to expand analytic functions in series of Bessel polynomials over complex contours, providing a basis for approximations in regions enclosing the origin. This non-classical setup also implies that the polynomials are complete in the space of holomorphic functions weighted by the exponential factors, enabling unique decompositions with controlled error bounds via the norm. Furthermore, it underscores the polynomials' utility in solving boundary value problems for linear differential equations with variable coefficients, where the complex weight captures essential singularity behaviors.[1]
Generalizations
Rodrigues Formula
Bessel polynomials admit a Rodrigues-type formula analogous to those for classical orthogonal polynomials, given byy_n(x) = 2^{-n} e^{2/x} \frac{d^n}{dx^n} \left( x^{2n} e^{-2/x} \right).This expression is derived by verifying orthogonality with respect to the weight function e^{-2/x}/x^2 on (0, \infty) via repeated integration by parts, confirming it matches the explicit hypergeometric representation y_n(x) = {}_2F_0(-n, n+1; ; -x/2). The constant term is normalized to unity, ensuring consistency with the standard definition. For small degrees, explicit computation yields y_0(x) = 1 and y_1(x) = x + 1, with leading coefficient 1.The formula highlights the distinction from classical orthogonal polynomials while connecting to broader special function theory. It facilitates demonstrations of properties such as the palindromic symmetry in the coefficients of y_n(x), where the coefficients read the same forward and backward.The Bessel polynomials are related to Laguerre polynomials through a limiting process, specifically y_n(x; a) = n! (-x/2)^n L_n^{(1-a-2n)}(2/x), where L_n^{(\alpha)} denotes the generalized Laguerre polynomial; however, the Bessel case remains distinct due to the non-polynomial weight and the specific parameter shift. This connection underscores the asymptotic behavior for large x, but the Rodrigues representations emphasize their unique role in non-standard orthogonality settings.[2]
Reversed and Associated Polynomials
The reversed Bessel polynomials \theta_n(x) are defined by the relation \theta_n(x) = x^n y_n(1/x), where y_n(x) denotes the standard Bessel polynomial of degree n. This transformation reverses the order of the coefficients in the power series expansion of y_n(x). An explicit expression for the reversed polynomials is\theta_n(x) = \sum_{k=0}^n \frac{(n+k)!}{k!(n-k)!} \frac{x^{n-k}}{2^k}.The reversed polynomials satisfy a second-order linear differential equation obtained by substitution into the standard Bessel differential equation, resulting in x^2 \theta_n''(x) + (2n+1 - x) x \theta_n'(x) + n(n+1) \theta_n(x) = 0. Recurrence relations for \theta_n(x) can be derived from those of y_n(x), including three-term relations such as (n+1) \theta_{n+1}(x) = (2n + 1 + x) \theta_n(x) - n \theta_{n-1}(x).The associated Bessel polynomials Y_n^{(\alpha)}(x) generalize the standard case by introducing a parameter \alpha \geq 0, defined explicitly asY_n^{(\alpha)}(x) = \sum_{k=0}^n \frac{(\alpha + n + k)!}{k! (\alpha + n - k)!} \left( \frac{x}{2} \right)^k.When \alpha = 0, this reduces to the standard Bessel polynomial y_n(x). The associated polynomials satisfy a modified differential equation x Y_n^{(\alpha)\prime\prime}(x) + (x + 2 + \alpha) Y_n^{(\alpha)\prime}(x) - n(n + 1 + \alpha) Y_n^{(\alpha)}(x) = 0, where the linear and constant terms in the first derivative coefficient and the eigenvalue are shifted by \alpha. Recurrence relations are similarly adjusted, for example, (n+1) Y_{n+1}^{(\alpha)}(x) = (2n + 1 + \alpha + x) Y_n^{(\alpha)}(x) - n(n + \alpha) Y_{n-1}^{(\alpha)}(x). For certain values of \alpha, the associated polynomials exhibit orthogonality properties with respect to a weight function w(x) = x^{\alpha} e^{-x} on (0, \infty), shifting the orthogonality measure from the standard case.These extensions, including reversed and associated forms, were developed in the late 1940s and 1950s, building on the initial introduction of Bessel polynomials, with contributions from R. P. Boas and others exploring their expansions and generalizations.
Generalized and q-Analogs
Generalized Bessel polynomials extend the classical form by introducing parameters \alpha and \beta, providing a flexible framework for studying orthogonal polynomials with modified weighting and differential properties. These polynomials are defined by the explicit sumY_n^{(\alpha,\beta)}(x) = \sum_{k=0}^n \frac{(\alpha + n + k)! \, (\beta + k)}{k! \, (\alpha + n - k)! \, (\beta + n - k)!} \left( \frac{x}{2} \right)^k.This parameterization allows for broader applications in approximation theory and special function analysis, where \alpha and \beta adjust the growth and orthogonality characteristics relative to the standard Bessel case (\alpha = \beta = 0).[4]A quantum analog, known as q-Bessel polynomials, replaces factorial terms with q-deformed structures using the q-Pochhammer symbol (a; q)_k = \prod_{j=0}^{k-1} (1 - a q^j). They are given byy_n(x; q) = \sum_{k=0}^n \frac{(q^{n+k}; q)_k}{(q; q)_k} \left( \frac{x}{2} \right)^k,which reduces to the classical Bessel polynomials as q \to 1. These q-analogs preserve key structural features while incorporating q-difference operators, making them suitable for discrete and quantum mechanical contexts.[4]The q-Bessel polynomials satisfy q-analog recurrences derived from three-term relations adapted to the q-calculus framework, such as y_{n+1}(x; q) = (x + [2n+1]_q) y_n(x; q) - _q [n+1]_q y_{n-1}(x; q), where _q = (1 - q^m)/(1 - q) denotes the q-number. Orthogonality holds with respect to a discrete measure or on q-contours in the complex plane, specifically \sum w_k y_m(x_k; q) y_n(x_k; q) = h_n \delta_{m,n}, where the weights w_k arise from q-integrals over deformed lattices. These properties facilitate their use in quantum group representations, particularly in models of the quantum plane and SU_q(2) symmetry, where they appear as matrix elements or characters in irreducible representations.[4][13]Recent theoretical advances include fractional extensions of q-Bessel polynomials, integrating Caputo or Riemann-Liouville fractional derivatives to solve non-integer order differential equations. For instance, papers from 2020 to 2023 explore fractional q-Bessel functions for operational matrices in numerical methods, emphasizing stability and convergence in q-deformed fractional calculus. Similar developments in Axioms journal address fractional q-extensions of orthogonal polynomials, including q-Bessel variants, with applications to q-integral transforms and hypergeometric series. The associated Bessel polynomials emerge as a special case when \beta = 1 in the generalized form, linking back to reversed variants.[4][14]
Zeros and Special Values
Distribution and Properties of Zeros
The zeros of the Bessel polynomials y_n(z) are all simple and located in the open left half-plane \operatorname{Re}(z) < 0 for n \geq 1. This placement ensures stability in applications such as filter design, where poles in the left half-plane are desirable. The simplicity of the zeros can be established using Rouché's theorem by comparing the polynomial to its dominant terms on suitable contours around potential multiple roots.[4][15]For n \geq 2, the zeros lie inside the cardioid given by r = 2(1 - \cos \theta ) in polar coordinates and outside the circle |z + 1| = 1. These bounds are derived using the argument principle applied to contours enclosing the respective regions, confirming no zeros outside the cardioid or inside the circle. The proof involves estimating the change in argument of y_n(z) along these boundaries, leveraging the recurrence relations and growth estimates of the polynomial coefficients. Numerical verification for small n supports these locations, with zeros clustering near the negative real axis but spreading into the complex plane as n increases.[15][16]Asymptotically, as n \to \infty, the zeros approach a limiting distribution along a curve in the left half-plane related to the level sets of the modified Bessel function of the second kind K_{\nu}(z). The zero of largest magnitude satisfies Luke's conjecture, which posits z_{n,1} \sim -n - \ln n + \ln(4\pi) + o(1), though this remains unproven despite supporting numerical evidence and partial results for related polynomials. This conjecture bounds the magnitude of all zeros, implying |z_{n,k}| \leq n + o(n) for k = 1, \dots, n.[4]Representative numerical examples for small degrees illustrate the distribution and increasing spread:
Degree n
Zeros (approximate)
1
-1
2
-0.500 ± 0.289i
3
-0.431 (real), -0.285 ± 0.272i
4
two complex pairs with \operatorname{Re}(z) < 0
5
one real ≈ -0.660; two complex pairs with \operatorname{Re}(z) < 0
These values, computed from the explicit polynomials, show the zeros confined to \operatorname{Re}(z) < 0, with the imaginary parts growing roughly as O(\sqrt{n}) and the spread widening for higher n, consistent with the asymptotic curve.[17][8]
Particular Values for Small Degrees
The Bessel polynomials of small degree provide concrete examples for illustrating their structure and properties. In the conventional definition where the constant term is normalized to 1, the explicit forms are monic in the reverse sense, with the lowest degree term being the constant 1 and the highest degree coefficient given by the double factorial (2n-1)!!. These polynomials satisfy y_n(0) = 1 for all n ≥ 0.[18]For n = 0 to 5, the explicit expressions are:
These forms can be verified using the explicit summation formula y_n(x) = \sum_{k=0}^n \frac{(n+k)!}{k!(n-k)!} \left( \frac{x}{2} \right)^k, which generates the coefficients directly.Evaluations at specific points further highlight their integer-valued nature in certain contexts. At x = 0, y_n(0) = 1 holds universally, as noted. At x = 2, the values are integers: y_0(2) = 1, y_1(2) = 3, y_2(2) = 19, y_3(2) = 193, y_4(2) = 2721, and y_5(2) = 49171. The ratios y_n(2)/n! yield 1, 3, 9.5, 32.1667, 113.375, and 409.7583 for n = 0 to 5, respectively; these ratios appear in expansions related to the generating function and aid in numerical verification of recurrence relations, such as the three-term recurrence y_{n+1}(x) = (x + 2n + 1) y_n(x) - n^2 y_{n-1}(x).[18][19]The leading coefficients and constant terms for the first 10 degrees are tabulated below, showcasing the pattern where the constant term remains 1 and the leading coefficient is (2n-1)!! for n ≥ 1:
These values facilitate direct computation and testing of properties like orthogonality or the differential equation x^2 y_n''(x) + (2x + 2) y_n'(x) = n(n + 1) y_n(x). For small n, the zeros of these polynomials (e.g., two complex conjugate zeros for n=2 at approximately -0.5 ± 0.289i) align with the general distribution discussed elsewhere.[18]
Bessel filters utilize Bessel polynomials to achieve a maximally flat group delay across the passband, preserving the time-domain shape of filtered signals with minimal distortion. The transfer function for an nth-order low-pass Bessel filter is expressed asH(s) = \frac{\theta_n(0)}{\theta_n(s)},where \theta_n(x) = x^n y_n(1/x) is the reverse Bessel polynomial and y_n(x) is the nth Bessel polynomial, ensuring the filter approximates an ideal delay transfer function e^{-s} in the low-frequency region. This design yields a smooth transition in the frequency response, prioritizing phase linearity over sharp cutoff attenuation.[20]The application of Bessel polynomials to filter design emerged in the late 1940s, with W. E. Thomson pioneering their use in 1949 for constructing delay networks featuring maximally flat frequency characteristics. In the 1950s, further advancements by Thomson and collaborators, including Louis Weinberg, incorporated reverse Bessel polynomials \theta_n(x) = x^n y_n(1/x) to synthesize stable filter prototypes with optimal delay properties. These efforts established Bessel filters as a cornerstone of analog electronic design during that era.[21]Bessel filters excel in scenarios requiring linear phase response, such as audio processing to avoid transient ringing and control systems for accurate signal timing. Their poles, derived from the zeros of \theta_n(s), lie in the left half of the complex plane, guaranteeing stability while maintaining near-constant group delay up to the passband edge. Unlike Butterworth or Chebyshev filters, Bessel designs sacrifice roll-off steepness for superior waveform preservation.[22]These filters are commonly realized through passive LC ladder networks, where inductors and capacitors alternate in a series-shunt configuration. Normalized prototype tables provide g-parameters for synthesis, scaled subsequently for specific cutoff frequencies and impedances; the normalization here assumes unit group delay at DC and 1 Ω termination. Representative values for low-pass prototypes up to fourth order are shown below, starting with a series inductor:
Order n
g_1 (L, H)
g_2 (C, F)
g_3 (L, H)
g_4 (C, F)
g_5 (load, Ω)
1
2.0000
1.0000
-
-
1.0000
2
1.5774
0.4226
1.0000
-
-
3
1.2550
0.5528
0.1922
1.0000
-
4
1.0598
0.5116
0.3181
0.1104
1.0000
For instance, a first-order filter uses a 2 H inductor in series followed by a 1 F capacitor to ground, delivering the desired delay characteristic. Higher-order realizations follow the same ladder topology, with values adjusted for practical components.[23]
In Physics and Differential Equations
Bessel polynomials find significant applications in modeling physical phenomena involving isotropic turbulence, where their orthogonality properties facilitate the representation of energy spectral functions. In particular, H. M. Srivastava demonstrated that certain orthogonal polynomials derived from Bessel polynomials can effectively describe the energy spectral functions for a family of isotropic turbulence fields, providing a mathematical framework to analyze the distribution of turbulent energy across different scales. This approach leverages the complete orthogonality relations of the polynomials over specific intervals with respect to exponential weight functions, enabling precise expansions of the spectral densities in turbulence models.Another key application lies in the realm of transform methods for solving differential equations, particularly through representations involving inverse Laplace transforms. The function e^{-t} / \sqrt{t} can be expressed via integrals incorporating the reversed Bessel polynomials \theta_n(x) = x^n y_n(1/x), which arise naturally in the expansion of solutions to certain linear differential equations with constant coefficients. This representation is useful for inverting Laplace transforms in problems where the transform domain involves square-root singularities, as seen in diffusion-like processes or heat conduction models. Such integrals stem from the generating function of the Bessel polynomials, \sum_{n=0}^{\infty} y_n(x) \frac{t^n}{n!} = \frac{1}{1 - x t + t^2}, allowing for series or integral forms that facilitate analytical solutions.In fractional differential equations, generalized Bessel polynomials, including q-analogs and fractional-order variants, serve as basis functions for approximating solutions to equations involving Caputo fractional derivatives, which model anomalous diffusion and viscoelastic behaviors in physical systems. For instance, numerical methods using fractional-order Bessel polynomials have been applied to solve nonlinear fractional-order population models, such as the logistic equation, where the Caputo derivative captures memory effects in growth dynamics. These polynomials provide orthogonal expansions that converge rapidly on finite intervals, improving accuracy for fractional orders between 0 and 1. Recent works from 2021 to 2023 highlight their efficacy in handling the nonlocal nature of Caputo operators, with applications to chaotic and fractal systems described by integro-differential equations.[24][4]Bessel polynomials also appear in quantum and wave mechanics, particularly in approximations for scattering problems in spherical coordinates. The zeros of reversed Bessel polynomials determine the poles of the S-matrix in partial-wave analysis, aiding in the study of resonance trapping and low-energy scattering phenomena in quantum systems. In acoustic wave scattering by spherical obstacles, time-domain solutions employ Bessel polynomials to expand transient fields, capturing the multipole contributions without relying on large-distance asymptotics. These approximations are valuable for modeling wave propagation in bounded domains, such as in quantum billiards or ultrasonic scattering, where the polynomials' relation to modified Bessel functions provides asymptotic insights for high angular momenta.[25]