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A007147 Number of self-dual 2-colored necklaces with 2n beads.
(Formerly M0319)
+0
3
1, 1, 2, 2, 4, 5, 9, 12, 23, 34, 63, 102, 190, 325, 612, 1088, 2056, 3771, 7155, 13364, 25482, 48175, 92205, 175792, 337594, 647326, 1246863, 2400842, 4636390, 8956060, 17334801, 33570816, 65108062, 126355336, 245492244, 477284182 (list; graph; listen)
OFFSET

1,3

REFERENCES

E. M. Palmer and R. W. Robinson, Enumeration of self-dual configurations, Pacific J. Math., 110 (1984), 203-221.

LINKS

Index entries for sequences related to necklaces

PROGRAM

(PARI) a(n)= (1/2) *(2^((n-1)\2)+sumdiv(n, k, (k%2)*eulerphi(k)*2^(n/k))/(2*n))

CROSSREFS

Cf. A000016, A016116. a(n)= (1/2) *(A016116(n-1)+A000016(n)).

Adjacent sequences: A007144 A007145 A007146 this_sequence A007148 A007149 A007150

Sequence in context: A001224 A102526 A050192 this_sequence A127968 A037026 A116651

KEYWORD

nonn,nice,easy

AUTHOR

njas

EXTENSIONS

More terms from Michael Somos

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Last modified October 7 08:31 EDT 2008. Contains 144667 sequences.


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