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A061891
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a(0) = 1; for n>0, a(n) = a(n-1) if n is already in the sequence, a(n) = a(n-1) + 3 otherwise.
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2
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1, 1, 4, 7, 7, 10, 13, 13, 16, 19, 19, 22, 25, 25, 28, 31, 31, 34, 37, 37, 40, 43, 43, 46, 49, 49, 52, 55, 55, 58, 61, 61, 64, 67, 67, 70, 73, 73, 76, 79, 79, 82, 85, 85, 88, 91, 91, 94, 97, 97, 100, 103, 103, 106, 109, 109, 112, 115, 115, 118, 121, 121, 124
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) = 2*n-1 if n == 1 (mod 3), 2*n if n == 2 (mod 3), 2*n + 1 if n == 0 (mod 3).
Differences are periodic with period 3.
a(n) = a(n-1) + a(n-3) - a(n-4).
G.f.: (2*x^3 + 3*x^2 + 1) / ((x - 1)^2*(x^2 + x +1)). (End)
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MATHEMATICA
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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