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 A211820 Number of (n+1)X(n+1) -10..10 symmetric matrices with every 2X2 subblock having sum zero and two, three or four distinct values 1
 220, 2310, 24280, 255510, 2692030, 28396180, 299874470, 3170351150, 33554420380, 355508955450, 3770437474330, 40027054951800, 425319384572170, 4523288178641650, 48144647645503520, 512829600353346710 (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..52 FORMULA Empirical: a(n) = 114*a(n-1) -5925*a(n-2) +185345*a(n-3) -3877235*a(n-4) +56925919*a(n-5) -597748066*a(n-6) +4472218575*a(n-7) -23123238043*a(n-8) +76099390938*a(n-9) -122330317169*a(n-10) -56591154105*a(n-11) +427598872995*a(n-12) +95124362391*a(n-13) -844404272684*a(n-14) -1140261335975*a(n-15) -728986025396*a(n-16) -278757941362*a(n-17) -67965466156*a(n-18) -10635078240*a(n-19) -1030171920*a(n-20) -55951200*a(n-21) -1296000*a(n-22) EXAMPLE Some solutions for n=3 ..7.-7..5..0..-10..1.-7..7....9..0..1.-6....8..0..2.-9....8.-6..2..1 .-7..7.-5..0....1..8.-2..2....0.-9..8.-3....0.-8..6..1...-6..4..0.-3 ..5.-5..3..2...-7.-2.-4..4....1..8.-7..2....2..6.-4.-3....2..0.-4..7 ..0..0..2.-7....7..2..4.-4...-6.-3..2..3...-9..1.-3.10....1.-3..7-10 CROSSREFS Sequence in context: A233626 A251322 A257353 * A215491 A258537 A027797 Adjacent sequences:  A211817 A211818 A211819 * A211821 A211822 A211823 KEYWORD nonn AUTHOR R. H. Hardin Apr 21 2012 STATUS approved

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Last modified January 24 09:32 EST 2022. Contains 350534 sequences. (Running on oeis4.)