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A109810 Number of permutations of the positive divisors of n, where every element is coprime to its adjacent elements. 6
1, 2, 2, 2, 2, 4, 2, 0, 2, 4, 2, 0, 2, 4, 4, 0, 2, 0, 2, 0, 4, 4, 2, 0, 2, 4, 0, 0, 2, 0, 2, 0, 4, 4, 4, 0, 2, 4, 4, 0, 2, 0, 2, 0, 0, 4, 2, 0, 2, 0, 4, 0, 2, 0, 4, 0, 4, 4, 2, 0, 2, 4, 0, 0, 4, 0, 2, 0, 4, 0, 2, 0, 2, 4, 0, 0, 4, 0, 2, 0, 0, 4, 2, 0, 4, 4, 4, 0, 2, 0, 4, 0, 4, 4, 4, 0, 2, 0, 0, 0, 2, 0, 2, 0, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Depends only on prime signature; a(A033942(n))=0; a(A037143(n))>0; a(A000430(n))=2; a(A006881(n))=4. [From Reinhard Zumkeller, May 24 2010]

LINKS

R. Zumkeller, Table of n, a(n) for n = 1..10000 [From Reinhard Zumkeller, May 24 2010]

FORMULA

a(1)=1, a(p) = 2, a(p^2) = 2, a(p*q) = 4 (where p and q are distinct primes), all other terms are 0.

EXAMPLE

The divisors of 6 are 1, 2, 3 and 6. Of the permutations of these integers,

only (6,1,2,3), (6,1,3,2), (2,3,1,6) and (3,2,1,6) are such that every pair of adjacent elements is coprime.

CROSSREFS

Cf. A178254. [From Reinhard Zumkeller, May 24 2010]

Sequence in context: A062816 A122857 A132003 * A122066 A053238 A227783

Adjacent sequences:  A109807 A109808 A109809 * A109811 A109812 A109813

KEYWORD

nonn

AUTHOR

Leroy Quet, Aug 16 2005

EXTENSIONS

Terms 17 to 59 from Diana L. Mecum, Jul 18 2008

More terms from David Wasserman, Oct 01 2008

STATUS

approved

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Last modified August 21 04:12 EDT 2014. Contains 245837 sequences.