A220613 Number of ways to reciprocally link elements of an nX7 array either to themselves or to exactly two horizontal and vertical neighbors, without consecutive collinear links.

1, 21, 85, 781, 4673, 34981, 233265, 1653229, 11354857, 79290645, 548972737, 3817993565, 26491584361, 184040357765, 1277739038129, 8873937621293, 61619077914745, 427911161639829, 2971470559369153, 20634790487757629

Offset: 1

Author

R. H. Hardin Dec 17 2012

Keywords

• nonn

Comments

Column 7 of A220614

Examples

			Some solutions for n=3 0=self 2=n 4=w 6=e 8=s (reciprocal directions total 10)
.68.48.00.68.48.00.00...68.48.68.48.00.00.00...00.00.00.68.48.00.00
.26.24.00.26.24.00.00...26.24.26.24.00.68.48...00.00.00.26.24.68.48
.00.00.00.00.00.00.00...00.00.00.00.00.26.24...00.00.00.00.00.26.24
		

Links

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

Formula

Empirical: a(n) = 5*a(n-1) +40*a(n-2) -144*a(n-3) -532*a(n-4) +1484*a(n-5) +3044*a(n-6) -6644*a(n-7) -8872*a(n-8) +13920*a(n-9) +13824*a(n-10) -13056*a(n-11) -10688*a(n-12) +4288*a(n-13) +3072*a(n-14)