Bárány's conjecture on extrema and Turán bounds for Bessel functions

Let u>0 u>0, and let JνJ_\nu be the Bessel function of the first kind of order u u. Write jν,nj_{\nu,n} for its positive zeros, set Ξ={jν,n:n1}\Xi=\{j_{\nu,n}:n\geq 1\}, and define

Φν(x)=Jν2(x)Jν1(x)Jν+1(x)Jν2(x)\Phi_\nu(x)=\frac{J_\nu^2(x)-J_{\nu-1}(x)J_{\nu+1}(x)}{J_\nu^2(x)}

for x(0,)Ξx\in(0,\infty)\setminus\Xi. The roots of Φν\Phi_\nu' are denoted by αν,n\alpha_{\nu,n}.

Bessel-function conjecture. The equation Φν(x)=0\Phi_\nu'(x)=0 has infinitely many roots, with

αν,n(jν,n,jν,n+1)\alpha_{\nu,n}\in(j_{\nu,n},j_{\nu,n+1})

for every n1n\geq1. Moreover, if βν,n=Φν(αν,n)\beta_{\nu,n}=\Phi_\nu(\alpha_{\nu,n}), then

Jν2(x)Jν1(x)Jν+1(x)>βν,nJν2(x)J_\nu^2(x)-J_{\nu-1}(x)J_{\nu+1}(x)>\beta_{\nu,n}J_\nu^2(x)

for x(jν,n,jν,n+1)x\in(j_{\nu,n},j_{\nu,n+1}), the sequence {βν,n}n0\{\beta_{\nu,n}\}_{n\geq0} is strictly increasing, and αν,0=jν,0=0\alpha_{\nu,0}=j_{\nu,0}=0 with βν,0=Φ(αν,0+)=1/(ν+1)\beta_{\nu,0}=\Phi(\alpha_{\nu,0}^+)=1/(\nu+1).

This conjecture proposes a sharp interval-by-interval improvement of Szász's Turán inequality, with the constants determined by the interior extrema of the normalized Turánian. Its resolution is not established in the supplied text.

Sources & referencesView supporting material

Primary source

Árpád Baricz and Tibor K. Pogány, “Turán determinants of Bessel functions”, arXiv:1101.4624 (2011).

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