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Either: The sequence beginning 1, 3... in which each number is equal to the sum of the previous two. It is closely related to the Fibonacci sequence. Or (less usually): The sequence of numbers u_n defined by:

un=4un−1−un−2.

Usually we take the first two values as u₀=2, u₁=4, so the sequence is:

2, 4, 14, 52, 194, 724, 2702, 10084, 37634 ...

Values in this sequence all satisfy the equation s²−3r²=4 for an integer value of r.