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A270545
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Number of equilateral triangle units forming perimeter of equilateral triangle.
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2
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1, 4, 9, 15, 21, 27, 33, 39, 45, 51, 57, 63, 69, 75, 81, 87, 93, 99, 105, 111, 117, 123, 129, 135, 141, 147, 153, 159, 165, 171, 177, 183, 189, 195, 201, 207, 213, 219, 225, 231, 237, 243, 249, 255, 261, 267, 273, 279, 285, 291, 297, 303, 309, 315
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OFFSET
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0,2
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COMMENTS
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This is the triangular equivalent of A008574 (square units forming perimeter of a square).
The height of each triangle is n+1 units.
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LINKS
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FORMULA
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a(0)=1 and a(1)=4; thereafter a(n) = (n+1)^2-(n-2)^2 = 6*n-3.
a(n) = 2*a(n-1)-a(n-2) for n>3. G.f.: (1+x)*(1+x+x^2) / (1-x)^2. - Colin Barker, Mar 20 2016
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MATHEMATICA
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CoefficientList[Series[(1 + x) (1 + x + x^2)/(1 - x)^2, {x, 0, 53}], x] (* Michael De Vlieger, Mar 21 2016 *)
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PROG
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(PARI) Vec((1+x)*(1+x+x^2)/(1-x)^2 + O(x^50)) \\ Colin Barker, Mar 20 2016
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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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