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A026475
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a(1)=1, a(2)=3, otherwise a(n) = least positive integer > a(n-1) and not a(i) + a(j) + a(k) for 1 <= i < j < k <= n.
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1
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1, 3, 4, 5, 6, 7, 19, 20, 21, 22, 36, 37, 38, 39, 53, 54, 55, 69, 70, 71, 72, 86, 87, 88, 102, 103, 104, 105, 119, 120, 121, 135, 136, 137, 138, 152, 153, 154, 168, 169, 170, 171, 185, 186, 187, 201, 202, 203, 204, 218, 219, 220, 234, 235, 236, 237, 251, 252, 253
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OFFSET
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1,2
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COMMENTS
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Without specifying a(2)=3, a(2) would be 2 and sequence would be A026471. - Robert Israel, Aug 27 2018
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LINKS
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FORMULA
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G.f.: x*(1 + 2*x + x^2 + x^3 + x^4 + x^5 + 12*x^6 - x^8 + 13*x^10 - 11*x^13 + 13*x^14) / ((1 - x)^2*(1 + x + x^2 + x^3 + x^4 + x^5 + x^6)).
a(n) = a(n-1) + a(n-7) - a(n-8) for n>15.
(End)
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MAPLE
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1, 3, 4, 5, 6, 7, 19, seq(op([20, 21, 22, 36, 37, 38, 39]+k*[33$7]), k=0..10); # Robert Israel, Aug 30 2018
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MATHEMATICA
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LinearRecurrence[{1, 0, 0, 0, 0, 0, 1, -1}, {1, 3, 4, 5, 6, 7, 19, 20, 21, 22, 36, 37, 38, 39, 53}, 60] (* Harvey P. Dale, Jan 04 2022 *)
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PROG
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(PARI) Vec(x*(1 + 2*x + x^2 + x^3 + x^4 + x^5 + 12*x^6 - x^8 + 13*x^10 - 11*x^13 + 13*x^14) / ((1 - x)^2*(1 + x + x^2 + x^3 + x^4 + x^5 + x^6)) + O(x^40)) \\ Colin Barker, Oct 10 2019
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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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EXTENSIONS
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More terms from Larry Reeves (larryr(AT)acm.org), Sep 25 2000
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STATUS
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approved
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