A392419 Number of mutual-visibility sets in the n X 3 grid graph.

7, 41, 190, 678, 2006, 5130, 11680, 24224, 46576, 84148, 144346, 237010, 374898, 574214, 855180, 1242652, 1766780, 2463712, 3376342, 4555102, 6058798, 7955490, 10323416, 13251960, 16842664, 21210284, 26483890, 32808010, 40343818, 49270366, 59785860, 72108980

Offset

1,1

Links

Andrew Howroyd, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (7,-21,35,-35,21,-7,1).

Formula

a(n) = (11*n^6 + 33*n^5 + 50*n^4 + 225*n^3 + 1379*n^2 - 1698*n + 720)/180 for n >= 3.

G.f.: x*(7 - 8*x + 50*x^2 - 36*x^3 + 60*x^4 - 36*x^5 + 4*x^6 + 4*x^7 - x^8)/(1 - x)^7.

Prog

(PARI) a(n) = if(n==1, 7, n==2, 41, (11*n^6 + 33*n^5 + 50*n^4 + 225*n^3 + 1379*n^2 - 1698*n + 720)/180)

Crossrefs

Row 3 of A392417.

Sequence in context: A034575 A237854 A219862 * A366996 A238991 A084779

Adjacent sequences: A392416 A392417 A392418 * A392420 A392421 A392422

Keyword

nonn,easy

Author

Andrew Howroyd, Jan 13 2026

Status

approved