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EXAMPLE
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a(2) = 2^2 = 4:
.
(1,2) (0,1)
/ \ / \
(2,2) (1,1) (0,0)
\ / \ /
(2,1) (1,0)
.
a(3) = 44:
.
(1,2,2)-(1,1,2)-(0,1,2)-(0,1,1)-(0,0,1)
/ X \ / X \
(2,2,2)-(2,1,2) (1,2,1)-(1,1,1)-(1,0,1) (0,1,0)-(0,0,0)
\ X / \ X /
(2,2,1)-(2,1,1)-(2,1,0)-(1,1,0)-(1,0,0)
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