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 A143280 #50 Feb 16 2025 08:33:08
%S A143280 3,0,5,9,4,0,7,4,0,5,3,4,2,5,7,6,1,4,4,5,3,9,4,7,5,4,9,9,2,3,3,2,7,8,
%T A143280 6,1,2,9,7,7,2,5,4,7,2,6,3,3,5,3,4,0,2,0,9,2,9,9,7,1,8,7,7,9,8,0,5,4,
%U A143280 4,2,8,1,9,6,8,4,6,1,3,5,3,5,7,4,8,1,8,5,7,4,4,8,3,4,9,7,8,2,8,3,1,5,0,1,5
%N A143280 Decimal expansion of m(2) = Sum_{n>=0} 1/n!!.
%C A143280 Also decimal expansion of Sum_{n>=1} n!!/n!. - _Michel Lagneau_, Dec 24 2011
%C A143280 Apart from the first digit, the same as A227569. - _Robert G. Wilson v_, Apr 09 2014
%H A143280 G. C. Greubel, <a href="/A143280/b143280.txt">Table of n, a(n) for n = 1..10000</a>
%H A143280 Michael Penn, <a href="https://www.youtube.com/watch?v=LGsXnm5sYow">Finding the closed form for a double factorial sum</a>, YouTube video, 2022.
%H A143280 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/DoubleFactorial.html">Double Factorial</a>
%H A143280 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ReciprocalMultifactorialConstant.html">Reciprocal Multifactorial Constant</a>
%F A143280 Equals sqrt(e) + sqrt((e*Pi)/2)*erf(1/sqrt(2)).
%e A143280 3.05940740534257614453947549923327861297725472633534020929971877980544281968...
%t A143280 RealDigits[ Sqrt[E] + Sqrt[E*Pi/2]*Erf[1/Sqrt[2]], 10, 105][[1]] (* or *)
%t A143280 RealDigits[ Sum[1/n!!, {n, 0, 125}], 10, 105][[1]] (* _Robert G. Wilson v_, Apr 09 2014 *)
%t A143280 RealDigits[Total[1/Range[0,200]!!],10,120][[1]] (* _Harvey P. Dale_, Apr 10 2022 *)
%o A143280 (PARI) default(realprecision, 100); exp(1/2)*(1 + sqrt(Pi/2)*(1-erfc(1/sqrt(2) ))) \\ _G. C. Greubel_, Mar 27 2019
%o A143280 (Magma) SetDefaultRealField(RealField(100)); R:= RealField(); Exp(1/2)*(1 + Sqrt(Pi(R)/2)*Erf(1/Sqrt(2) )); // _G. C. Greubel_, Mar 27 2019
%o A143280 (Sage) numerical_approx(exp(1/2)*(1 + sqrt(pi/2)*erf(1/sqrt(2))), digits=100) # _G. C. Greubel_, Mar 27 2019
%Y A143280 Cf. A227569.
%Y A143280 Cf. A006882 (n!!), this sequence (m(2)), A288055 (m(3)), A288091 (m(4)), A288092 (m(5)), A288093 (m(6)), A288094 (m(7)), A288095 (m(8)), A288096 (m(9)).
%K A143280 nonn,cons
%O A143280 1,1
%A A143280 _Eric W. Weisstein_, Aug 04 2008