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A177282
Number of permutations of 2 copies of 1..n with all adjacent differences <= 1 in absolute value.
2
1, 1, 6, 12, 26, 48, 86, 148, 250, 416, 686, 1124, 1834, 2984, 4846, 7860, 12738, 20632, 33406, 54076, 87522, 141640, 229206, 370892, 600146, 971088, 1571286, 2542428, 4113770, 6656256, 10770086, 17426404, 28196554, 45623024, 73819646, 119442740, 193262458
OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000 (terms n=1..197 from R. H. Hardin)
FORMULA
a(n) = (2n)!/2^n = A000680(n) for n<=2.
MAPLE
a:= proc(n) option remember; `if`(n<4, [1$2, 6, 12][n+1],
((8*n-31)*a(n-1) -(4*n-19)*a(n-2) -(3*n-10)*a(n-3)
+(2*n-10)*a(n-4)) / (3*n-11))
end:
seq(a(n), n=0..40); # Alois P. Heinz, Jan 14 2016
CROSSREFS
Column k=2 of A331562.
Cf. A000680.
Sequence in context: A065106 A264008 A283221 * A283139 A242843 A232935
KEYWORD
nonn,easy
AUTHOR
R. H. Hardin, May 06 2010
STATUS
approved