A116980 Number of ways to arrange integers 1...n so that the sum of each adjacent pair is a triangular number, not counting reversals.

1, 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

Offset: 0

Author

Giovanni Resta, Apr 01 2006

Keywords

• nonn

Examples

			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.
		

Crossrefs

Cf. A051239.

Extensions

a(29)-a(33) from Donovan Johnson, Aug 27 2010

a(0)=1 prepended by Alois P. Heinz, Oct 05 2020