×

Writing \(\pi\) as sum of arctangents with linear recurrent sequences, golden mean and Lucas numbers. (English) Zbl 1360.11029

Let \(A(x)=\arctan(x)\). Further, for any real \(x\), let \[ u_n(x)=\frac{\alpha(x)^n+\beta(x)^n}{2}\quad\text{and}\quad v_n(x)=\frac{\alpha(x)^n-\beta(x)^n}{\alpha-\beta}, \] where \((\alpha(x),\beta(x))=(1+ix, 1-ix)\), \(i={\sqrt{-1}}\). The authors show that for any positive integer \(n\) and any real number \(x\neq \pm 1\), we have \(A^{n}(1/x)=v_n(x)/u_n(x)\) (Theorem 1.3) and \(A^n(x)=(-1)^{n+1} (v_n(x)/u_n(x))^{(-1)^n}\) (Theorem 1.4). As a consequence, they exhibit identities such as \[ n\arctan \left(\frac{1}{x}\right)+\arctan\left(\frac{u_n(x)-v_n(x)}{u_n(x)+v_n(x)}\right)=\frac{\pi}{4}+k(n,x)\pi, \] where \(k(n,x)\) is some suitable integer. Similarly, they prove that \[ 2\arctan (-x\pm {\sqrt{1+x^2}})+\arctan(x)=\pm \frac{\pi}{2} \] holds for every real number \(x\). Taking \(x=L_{2k+1}/2\), where \(\{L_m\}_{m\geq 0}\) is the Lucas companion of the Fibonacci sequence, they get formulas such as \[ \frac{\pi}{2}=\arctan \left(\frac{L_{2k+1}}{2}\right)+2\arctan\left(\frac{1}{\phi^{2k+1}}\right), \] where \(\phi=(1+{\sqrt{5}})/2\) is the Golden mean. The proofs are by induction.

MSC:

11B37 Recurrences
11B39 Fibonacci and Lucas numbers and polynomials and generalizations

References:

[1] Barbero S., Fibonacci Quart. 48 pp 348– (2010)
[2] DOI: 10.4169/193009709X470416 · Zbl 1229.11164 · doi:10.4169/193009709X470416
[3] Chan H. C., Fibonacci Quart. 44 pp 141– (2006)
[4] Chan H. C., Fibonacci Quart. 48 pp 32– (2009)
[5] Hwang C.-L., Math. Gaz. 88 pp 270– (2004) · doi:10.1017/S0025557200175060
[6] Lidl R., Pitman Monographs and Surveys in Pure and Applied Mathematics 65, in: Dickson Polynomials (1993)
[7] DOI: 10.1142/S1793042109002493 · Zbl 1231.11035 · doi:10.1142/S1793042109002493
[8] DOI: 10.1007/978-94-009-0223-7_33 · doi:10.1007/978-94-009-0223-7_33
[9] Rédei L., Acta Sci. Math. (Szeged) 11 pp 85– (1946)
[10] DOI: 10.1007/BF00384331 · Zbl 0765.01003 · doi:10.1007/BF00384331
[11] Wrench J. W., Math. Teacher 53 pp 644– (1960)
This reference list is based on information provided by the publisher or from digital mathematics libraries. Its items are heuristically matched to zbMATH identifiers and may contain data conversion errors. In some cases that data have been complemented/enhanced by data from zbMATH Open. This attempts to reflect the references listed in the original paper as accurately as possible without claiming completeness or a perfect matching.