

A177282


Number of permutations of 2 copies of 1..n with all adjacent differences <= 1 in absolute value.


1



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
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

0,3


COMMENTS

a(n) = (2n)!/2^n for n<=2.


LINKS

R. H. Hardin and Alois P. Heinz, Table of n, a(n) for n = 0..1000 (terms n=1..197 from R. H. Hardin)


MAPLE

a:= proc(n) option remember; `if`(n<4, [1$2, 6, 12][n+1],
((8*n31)*a(n1) (4*n19)*a(n2) (3*n10)*a(n3)
+(2*n10)*a(n4)) / (3*n11))
end:
seq(a(n), n=0..40); # Alois P. Heinz, Jan 14 2016


CROSSREFS

Sequence in context: A065106 A264008 A283221 * A283139 A242843 A232935
Adjacent sequences: A177279 A177280 A177281 * A177283 A177284 A177285


KEYWORD

nonn,easy


AUTHOR

R. H. Hardin, May 06 2010


STATUS

approved



