This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A385405 #13 Jul 28 2025 08:43:49
%S A385405 2,0,9,9,8,7,1,7,0,8,0,7,0,1,3,0,3,4,6,9,7,2,4,8,3,6,9,5,2,0,8,5,0,7,
%T A385405 2,2,4,5,8,5,9,3,3,6,4,1,1,5,3,8,5,4,7,7,3,5,6,6,5,5,7,2,0,1,2,2,2,9,
%U A385405 4,9,4,9,7,6,2,0,2,4,1,5,2,8,0,7,8,4,7,0,0,4,4,3,1,1,5,9,3,6,2,9,8,2,5,7,6
%N A385405 Decimal expansion of 1/(2*cosh(1)^2).
%H A385405 Hideyuki Ohtsuka, <a href="https://www.jstor.org/stable/10.4169/amer.math.monthly.124.5.465">Problem 11978</a>, Problems and Solutions, The American Mathematical Monthly, Vol. 124, No. 5 (2017), p. 465; <a href="https://www.jstor.org/stable/48662488">A Sum of Hyperbolic Cosines of Fibonacci Numbers</a>, Solution to Problem 11978 by Kyle Gatesman, ibid., Vol. 126, No. 2 (2019), p. 185.
%F A385405 Equals 1/(2*A073743^2).
%F A385405 Equals Sum_{n>=0} (-1)^n/(cosh(Fibonacci(n)) * cosh(Fibonacci(n+3))) (Ohtsuka, 2017).
%e A385405 0.20998717080701303469724836952085072245859336411538...
%t A385405 RealDigits[1/(2*Cosh[1]^2), 10, 120][[1]]
%o A385405 (PARI) 1/(2*cosh(1)^2)
%Y A385405 Cf. A000045, A073743.
%K A385405 nonn,cons,easy
%O A385405 0,1
%A A385405 _Amiram Eldar_, Jul 25 2025