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A046994
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Number of Greek-key tours on a 3 X n board; i.e. self-avoiding walks on 3 X n grid starting in top left corner.
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5
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1, 3, 8, 17, 38, 78, 164, 332, 680, 1368, 2768, 5552, 11168, 22368, 44864, 89792, 179840, 359808, 720128, 1440512, 2882048, 5764608, 11531264, 23063552, 46131200, 92264448, 184537088, 369078272, 738172928, 1476354048
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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REFERENCES
| Posting by Thomas Womack (mert0236(AT)sable.ox.ac.uk) to sci.math newsgroup, Apr 21 1999.
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FORMULA
| a(1) = 1; a(2m) = sum_{i = 2...2m-1} a(i) + 3*2^(m-1); a(2m+1) = sum_{i = 2...2m}a(i) + 5*2^(m-1).
a(n) = 11*2^(n-3) - (4 + (-1)^n)*(2^((1/4)*(2n - 7 - (-1)^n))), n >= 2 - Nathaniel Johnston (nathaniel(AT)nathanieljohnston.com), Feb 03 2006
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EXAMPLE
| On a 3 X 3 board labeled 123 456 789 (reading across rows), 125478963 is such a tour.
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CROSSREFS
| Cf. A046995.
Sequence in context: A097391 A202554 A034481 * A058811 A101822 A088589
Adjacent sequences: A046991 A046992 A046993 * A046995 A046996 A046997
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KEYWORD
| nonn,walk
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AUTHOR
| Antreas P. Hatzipolakis (xpolakis(AT)otenet.gr)
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EXTENSIONS
| More terms and formula from Hugo van der Sanden (hv(AT)crypt.org)
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