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A201975 Number of n X 2 0..3 arrays with rows and columns lexicographically nondecreasing read forwards and nonincreasing read backwards. 1
4, 30, 72, 131, 208, 304, 420, 557, 716, 898, 1104, 1335, 1592, 1876, 2188, 2529, 2900, 3302, 3736, 4203, 4704, 5240, 5812, 6421, 7068, 7754, 8480, 9247, 10056, 10908, 11804, 12745, 13732, 14766, 15848, 16979, 18160, 19392, 20676, 22013, 23404, 24850 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Column 2 of A201981.

LINKS

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

FORMULA

Empirical: a(n) = (1/6)*n^3 + 7*n^2 + (23/6)*n - 7.

Conjectures from Colin Barker, May 25 2018: (Start)

G.f.: x*(4 + 14*x - 24*x^2 + 7*x^3) / (1 - x)^4.

a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4) for n>4.

(End)

EXAMPLE

Some solutions for n=9.

..0..3....1..3....0..3....0..3....1..3....0..3....0..3....1..2....0..3....0..1

..0..3....1..3....0..3....0..3....2..2....0..3....1..2....3..1....0..3....3..0

..1..2....1..3....0..3....0..3....2..2....0..3....1..2....3..1....1..2....3..0

..1..2....1..3....0..3....0..3....2..2....0..3....1..2....3..1....2..1....3..0

..1..2....1..3....2..0....0..3....2..2....2..1....1..2....3..1....2..1....3..0

..2..0....1..3....2..0....1..2....2..2....2..1....1..2....3..1....2..1....3..0

..2..0....2..2....2..0....1..2....2..2....2..1....1..2....3..1....3..0....3..0

..2..0....3..1....2..0....1..2....3..0....2..1....3..1....3..1....3..0....3..0

..2..0....3..1....2..0....2..1....3..0....2..1....3..1....3..1....3..0....3..0

CROSSREFS

Cf. A201981.

Sequence in context: A268037 A248528 A296247 * A109670 A014697 A211624

Adjacent sequences:  A201972 A201973 A201974 * A201976 A201977 A201978

KEYWORD

nonn

AUTHOR

R. H. Hardin, Dec 07 2011

STATUS

approved

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Last modified December 14 23:10 EST 2018. Contains 318141 sequences. (Running on oeis4.)