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A211493 Number of (n+1)X(n+1) -4..4 symmetric matrices with every 2X2 subblock having sum zero and two or three distinct values 1
40, 96, 214, 446, 920, 1852, 3716, 7414, 14772, 29486, 58836, 117908, 236354, 475986, 959170, 1940794, 3930106, 7985174, 16237818, 33106698, 67555856, 138133826, 282669656, 579365960, 1188357854, 2440574368, 5015738686, 10318671714 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Symmetry and 2X2 block sums zero implies that the diagonal x(i,i) are equal modulo 2 and x(i,j)=(x(i,i)+x(j,j))/2*(-1)^(i-j)

LINKS

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

FORMULA

Empirical: a(n) = 4*a(n-1) +8*a(n-2) -47*a(n-3) -13*a(n-4) +224*a(n-5) -60*a(n-6) -553*a(n-7) +282*a(n-8) +745*a(n-9) -465*a(n-10) -524*a(n-11) +351*a(n-12) +165*a(n-13) -112*a(n-14) -18*a(n-15) +12*a(n-16)

EXAMPLE

Some solutions for n=3

.-3..1.-1..2....0..2..2.-2...-4..2.-4..0...-1..0.-1..2...-3..3..0..3

..1..1.-1..0....2.-4..0..0....2..0..2..2....0..1..0.-1....3.-3..0.-3

.-1.-1..1..0....2..0..4.-4...-4..2.-4..0...-1..0.-1..2....0..0..3..0

..2..0..0.-1...-2..0.-4..4....0..2..0..4....2.-1..2.-3....3.-3..0.-3

CROSSREFS

Sequence in context: A038466 A279689 A063310 * A185791 A092889 A235280

Adjacent sequences:  A211490 A211491 A211492 * A211494 A211495 A211496

KEYWORD

nonn

AUTHOR

R. H. Hardin Apr 13 2012

STATUS

approved

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Last modified May 25 19:19 EDT 2022. Contains 354071 sequences. (Running on oeis4.)