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%I #28 Jan 26 2024 16:02:29
%S 1,1,2,4,8,10,14,18,22,18,12,12,12,12,12,12,12,12,12,12,12,12,12,12,
%T 12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,
%U 12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12
%N Number of log-concave permutations of {1,...,n}.
%C a(n) = 12 for n >= 10, since for these n the only log-concave permutations of {1,...,n} are (1,2,...,n), (1,2,4,5,...,n,3), (1,2,4,6,...,7,5,3), (1,3,4,...,n,2), (1,3,5,...,6,4,2), (1,n,...,3,2), and the reverses of these.
%H Pontus Andersson (von Brömssen), <a href="/A301454/a301454.pdf">Log-concave permutations</a>, Manuscript, 2002.
%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/LogarithmicallyConcaveSequence.html">Logarithmically Concave Sequence</a>.
%H <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (1).
%F G.f.: (6*x^10 +4*x^9 -4*x^8 -4*x^7 -4*x^6 -2*x^5 -4*x^4 -2*x^3 -x^2 -1)/(x-1).
%Y Cf. A069916, A301454.
%K easy,nonn
%O 0,3
%A _Pontus von Brömssen_, Mar 12 2018
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