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%I #9 Aug 27 2024 09:57:44
%S 2,1,9,4,5,4,5,2,0,6,2,2,4,7,2,1,3,0,0,8,2,4,4,7,0,5,9,3,8,7,9,1,3,0,
%T 3,6,1,2,9,4,6,3,1,5,3,3,0,0,3,8,4,9,5,2,3,1,0,8,2,5,3,7,9,2,0,8,4,4,
%U 3,5,5,6,5,5,0,5,1,0,0,6,5,4,7,9,0,3
%N Decimal expansion of Sum_{k>=0} (-1)^k (k+1)! / (2k)!.
%F Equals (6 e^(1/4) - 5 Sqrt(Pi) erfi(1/2))/(8 e^(1/4)).
%F Equals 1F1(2;1/2;-1/4). - _R. J. Mathar_, Aug 27 2024
%e 0.219454520622472130082447059387913036129463153300...
%t s = Sum[(-1)^k (k + 1)!/(2 k)!, {k, 0, Infinity}]
%t d = N[s, 100]
%t First[RealDigits[d]]
%Y Cf. A371937.
%K nonn,cons
%O 0,1
%A _Clark Kimberling_, Apr 27 2024