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A195033
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Multiples of 21 and of 20 interleaved: a(2n-1) = 21n, a(2n) = 20n.
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7
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21, 20, 42, 40, 63, 60, 84, 80, 105, 100, 126, 120, 147, 140, 168, 160, 189, 180, 210, 200, 231, 220, 252, 240, 273, 260, 294, 280, 315, 300, 336, 320, 357, 340, 378, 360, 399, 380, 420, 400, 441, 420, 462, 440, 483, 460, 504, 480, 525, 500, 546, 520, 567, 540
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OFFSET
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1,1
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COMMENTS
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a(n) is also the length of the n-th edge of a square spiral in which the first two edges are the legs of the primitive Pythagorean triple [21, 20, 29]. Zero together with partial sums give A195034, the vertices of the spiral.
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LINKS
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FORMULA
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G.f.: x*(21+20*x)/((1-x)^2*(1+x)^2).
a(n) = 2*a(n-2) - a(n-4). (End)
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MATHEMATICA
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Riffle[21*#, 20*#] & [Range[50]] (* Paolo Xausa, Mar 21 2024 *)
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PROG
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(Magma) &cat[[21*n, 20*n]: n in [1..27]]; // Bruno Berselli, Sep 29 2011
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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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STATUS
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approved
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