OFFSET
0,3
COMMENTS
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.
LINKS
Pontus Andersson (von Brömssen), Log-concave permutations, Manuscript, 2002.
Eric Weisstein's World of Mathematics, Logarithmically Concave Sequence.
Index entries for linear recurrences with constant coefficients, signature (1).
FORMULA
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).
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Pontus von Brömssen, Mar 12 2018
STATUS
approved