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A116980
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Number of ways to arrange integers 1...n so that the sum of each adjacent pair is a triangular number, not counting reversals.
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0
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1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 4, 19, 23, 16, 43, 59, 66, 127, 492, 886, 964, 2595, 11426, 36780, 78070, 131232, 423402, 1302893, 3356226, 6673924, 16604441, 32195439, 69328860
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,11
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EXAMPLE
| a(9)=1, since {3,7,8,2,4,6,9,1,5} and its reversal are the only permutations
of 1..9 with the given property. Here 3+7, 7+8, 8+2, 4+6, 6+9, 9+1 and 1+5 are all triangular numbers.
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CROSSREFS
| Cf. A051239.
Sequence in context: A043056 A012879 A072178 * A022135 A192193 A028564
Adjacent sequences: A116977 A116978 A116979 * A116981 A116982 A116983
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KEYWORD
| nonn
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AUTHOR
| Giovanni Resta (g.resta(AT)iit.cnr.it), Apr 01 2006
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EXTENSIONS
| a(29)-a(33) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Aug 27 2010
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