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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*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

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

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Last modified January 19 00:40 EST 2020. Contains 331030 sequences. (Running on oeis4.)