A006720
Somos-4 sequence: $a(0)=a(1)=a(2)=a(3)=1$; for $n\ge 4$, $a(n)=(a(n-1)a(n-3)+a(n-2{)}^2)/a(n-4)$.

1, 1, 1, 1, 2, 3, 7, 23, 59, 314, 1529, 8209, 83313, 620297, 7869898, 126742987, 1687054711, 47301104551, 1123424582771, 32606721084786, 1662315215971057, 61958046554226593, 4257998884448335457, 334806306946199122193, ...

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