A220684 Number of ways to reciprocally link elements of an nX4 array either to themselves or to exactly two horizontal, vertical or antidiagonal neighbors.

1, 37, 745, 15452, 312603, 6349886, 128995678, 2620320833, 53226950200, 1081210102382, 21962842195925, 446135673339403, 9062444632025696, 184087280486642815, 3739402357364779367, 75959240384539034764

Offset: 1

Author

R. H. Hardin Dec 17 2012

Keywords

• nonn

Comments

Column 4 of A220688

Examples

			Some solutions for n=3 0=self 2=n 3=ne 4=w 6=e 7=sw 8=s (reciprocal directions total 10)
.00.67.48.78...68.46.46.47...00.00.00.78...00.00.68.48...68.46.46.47
.38.78.23.27...26.48.37.00...00.67.34.27...00.68.24.27...28.00.38.00
.23.26.34.00...00.23.00.00...36.46.34.00...00.26.34.00...26.46.24.00
		

Links

• R. H. Hardin, Table of n, a(n) for n = 1..210

Formula

Empirical: a(n) = 15*a(n-1) +89*a(n-2) +365*a(n-3) +522*a(n-4) -1997*a(n-5) -10213*a(n-6) -23837*a(n-7) -26018*a(n-8) +7514*a(n-9) +19116*a(n-10) -3306*a(n-11) -5327*a(n-12) +2212*a(n-13) +316*a(n-14) -285*a(n-15) +12*a(n-17) -a(n-18)