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

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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:n≥1}\Xi=\{j_{\nu,n}:n\geq 1\}, and define

Φν(x)=Jν2(x)−Jν−1(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 n≥1n\geq1. Moreover, if βν,n=Φν(αν,n)\beta_{\nu,n}=\Phi_\nu(\alpha_{\nu,n}), then

Jν2(x)−Jν−1(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}n≥0\{\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.

References

Primary source

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

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