English

Higher derivatives of the inverse tangent function and a summation formula involving binomial coefficients

Classical Analysis and ODEs 2018-10-22 v2

Abstract

In 2017, O. Deiser and C. Lasser obtained an explicit formula for the nn-th derivative of the inverse tangent function. We calculate this derivative by a different method based on Fa\`a di Bruno's formula. Comparing the two results leads to the following identity for binomial coefficients: i=m[n/2](1)i4i(im)(nii)=(1)m2n(n+12m+1),\sum_{i=m}^{[n/2]}\frac{(-1)^i}{4^i}\binom{i}{m}\binom{n-i}{i}=\frac{(-1)^m}{2^n}\binom{n+1}{2m+1}, where n,mN0n,m\in \mathbb{N}_0 and m[n/2]m\leq [n/2]. As was pointed out to the author by C. Krattenthaler, this formula is a special case of Gau\ss's formula for the hypergeometric function 2F1_2F_1.

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Cite

@article{arxiv.1809.08184,
  title  = {Higher derivatives of the inverse tangent function and a summation formula involving binomial coefficients},
  author = {Jan-David Hardtke},
  journal= {arXiv preprint arXiv:1809.08184},
  year   = {2018}
}

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8 pages