A225305 Number of nX4 -1,1 arrays such that the sum over i=1..n,j=1..4 of i*x(i,j) is zero and rows are nondecreasing (ways to put 4 thrusters pointing east or west at each of n positions 1..n distance from the hinge of a south-pointing gate without turning the gate)

1, 3, 9, 31, 113, 443, 1787, 7445, 31593, 136351, 596187, 2636015, 11763569, 52920569, 239731317, 1092632909, 5006816043, 23053197067, 106601983853, 494856940025, 2305237337415, 10772988573393, 50491926308985

Offset

1,2

Comments

Column 4 of A225310

Links

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

Example

Some solutions for n=4
.-1..1..1..1...-1.-1.-1.-1...-1.-1..1..1...-1.-1.-1..1...-1.-1..1..1
.-1.-1.-1..1...-1.-1.-1.-1...-1.-1..1..1...-1.-1.-1..1...-1.-1.-1..1
.-1.-1.-1..1....1..1..1..1...-1.-1..1..1...-1..1..1..1...-1.-1.-1.-1
.-1..1..1..1...-1.-1..1..1...-1.-1..1..1...-1.-1..1..1....1..1..1..1

Crossrefs

Sequence in context: A052709 A049179 A049154 * A365113 A110136 A202180

Adjacent sequences: A225302 A225303 A225304 * A225306 A225307 A225308

Keyword

nonn

Author

R. H. Hardin May 05 2013

Status

approved