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%I #21 Dec 17 2021 17:28:17
%S 1,1,0,1,5,1,0,1,2,3,0,1,0,1,0,9,2,15,0,1,2,9,0,1,0,7,0,15,2,1,0,1,0,
%T 27,0,1,0,25,0,21,2,11,0,1,45,33,0,25,0,39,0,27,0,49,0,1,57,15,0,1,0,
%U 1,0,33,2,51,0,35,2,9,0,1,0,19,0,39,77,65,0,1,81,63,0,1,0,33,0,45,0
%N Quantum factorials: (n-1)!! - (n-2)!! (mod n).
%D S. P. Hurd and J. S. McCranie, Quantum factorials. Proceedings of the Twenty-fifth Southeastern International Conference on Combinatorics, Graph Theory and Computing (Boca Raton, FL, 1994). Congr. Numer. 104 (1994), 19-24.
%H T. D. Noe, <a href="/A007912/b007912.txt">Table of n, a(n) for n = 3..2000</a>
%H S. P. Hurd and J. S. McCranie, <a href="https://www.researchgate.net/publication/265637612_Quantum_factorials">Quantum factorials</a>, 1994.
%F a(n) = 0 iff n is odd and not a prime congruent to 3 modulo 4. - _Charlie Neder_, Feb 24 2019
%p a:= n-> (d-> irem(d(n-1)-d(n-2), n))(doublefactorial):
%p seq(a(n), n=3..100); # _Alois P. Heinz_, Dec 17 2021
%t Table[Mod[(n-1)!!-(n-2)!!,n],{n,3,100}] (* _Harvey P. Dale_, Aug 07 2012 *)
%Y Cf. A006882, A007911.
%K nonn,easy,nice
%O 3,5
%A _J. H. Conway_
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