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FORMULA
| a(n) = Sum_{k=0..floor(n/2)} n!/(n-2*k)!.
a(n) = n*(n-1)*a(n-2) + 1. - Vladeta Jovovic (vladeta(AT)eunet.rs), Aug 24 2004
a(n) = (A000522(n)+(-1)^n*A000166(n))/2. - Vladeta Jovovic (vladeta(AT)eunet.rs), Aug 24 2004
a(n)=sum{k=0..n, binomial(n, k)(1+(-1)^k)k!/2} Binomial transform of A010050 (with interpolated zeros). - Paul Barry (pbarry(AT)wit.ie), Sep 14 2004
a(n) = Sum[P(n, k)[1, 0, 1, 0, 1, 0...](k), {k, 0, n}]. - Ross La Haye (rlahaye(AT)new.rr.com), Aug 29 2005
a(n) = (1/(2*exp(1))) * [int(t^n*exp(1-abs(1-t)), t=0..2) + int([(2+t)^n+(-t)^n] * exp(-t), t=0..infinity)]- Groux Roland, Jan 15 2011
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