The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A220720 Number of ways to reciprocally link elements of an n X 3 array either to themselves or to exactly two horizontal, diagonal or antidiagonal neighbors. 1
 1, 7, 30, 126, 534, 2275, 9667, 41089, 174660, 742428, 3155820, 13414405, 57020437, 242375995, 1030264362, 4379330826, 18615162402, 79127219431, 336345003127, 1429697162869, 6077194423824, 25832248272072, 109804788896376 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Column 3 of A220725. LINKS R. H. Hardin, Table of n, a(n) for n = 1..210 FORMULA Empirical: a(n) = 3*a(n-1) + 4*a(n-2) + 5*a(n-3) + 3*a(n-4) - 2*a(n-5). Empirical g.f.: x*(1 + 4*x + 5*x^2 + 3*x^3 - 2*x^4) / ((1 + x)*(1 - 4*x - 5*x^3 + 2*x^4)). - Colin Barker, Aug 02 2018 EXAMPLE Some solutions for n=3 0=self 1=nw 3=ne 4=w 6=e 7=sw 9=se (reciprocal directions total 10): .69.49.00...69.47.00...00.00.00...00.79.00...69.47.00...00.69.47...00.79.00 .00.17.17...39.16.47...00.69.47...39.69.14...39.19.00...00.36.14...36.49.17 .36.34.00...00.13.00...00.36.14...00.16.14...00.16.14...00.00.00...00.36.14 CROSSREFS Cf. A220725. Sequence in context: A037709 A037611 A171472 * A024311 A210100 A297080 Adjacent sequences:  A220717 A220718 A220719 * A220721 A220722 A220723 KEYWORD nonn AUTHOR R. H. Hardin, Dec 18 2012 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 4 02:18 EST 2020. Contains 338921 sequences. (Running on oeis4.)