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 A020784 #48 Jul 05 2025 19:26:16
%S A020784 1,9,2,4,5,0,0,8,9,7,2,9,8,7,5,2,5,4,8,3,6,3,8,2,9,2,6,8,3,3,9,8,5,8,
%T A020784 1,8,5,4,9,2,0,0,5,8,3,7,5,6,7,0,8,9,5,8,6,7,2,8,6,7,4,4,2,1,6,1,3,2,
%U A020784 5,8,9,0,7,6,7,6,4,4,4,4,8,5,6,4,5,7,1,7,9,8,5,2,8,5,8,3,1,7,5
%N A020784 Decimal expansion of 1/sqrt(27).
%C A020784 This is the minimum ripple factor for a third-order Chebyshev filter for which the generalized reflectionless topology needs no negative elements. - _Matthew A. Morgan_, Oct 18 2017
%D A020784 Steven R. Finch, Mathematical Constants, Encyclopedia of Mathematics and its Applications, vol. 94, Cambridge University Press, 2003, Sections 8.4.3 and 8.16, pp. 495, 527.
%H A020784 Ivan Panchenko, <a href="/A020784/b020784.txt">Table of n, a(n) for n = 0..1000</a>
%H A020784 <a href="/index/Al#algebraic_02">Index entries for algebraic numbers, degree 2</a>.
%F A020784 Equals Sum_{k>=0} binomial(2*k,k) * k/16^k. - _Amiram Eldar_, Aug 02 2020
%F A020784 Equals sqrt(3)/9. - _Stefano Spezia_, Dec 24 2024
%F A020784 Equals 1/A010482 = A020760/3 = sqrt(A021031) = A073010/Pi = A212886/2. - _Hugo Pfoertner_, Dec 24 2024
%e A020784 0.1924500897298752548363829268339858185492005837567089586728674....
%t A020784 RealDigits[1/Sqrt[27],10,200][[1]] (* _Vladimir Joseph Stephan Orlovsky_, May 30 2010 *)
%t A020784 realDigitsRecip[Sqrt[27]] (* The realDigitsRecip program is at A021200 *) (* _Harvey P. Dale_, Jul 05 2025 *)
%o A020784 (PARI) 1/sqrt(27) \\ _G. C. Greubel_, Feb 15 2018
%o A020784 (Magma) 1/Sqrt(27); // _G. C. Greubel_, Feb 15 2018
%Y A020784 Cf. A002194, A010482, A020760, A021031, A073010, A212886.
%K A020784 nonn,cons,easy
%O A020784 0,2
%A A020784 _N. J. A. Sloane_