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A195312
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Multiples of 9 and odd numbers interleaved.
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23
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0, 1, 9, 3, 18, 5, 27, 7, 36, 9, 45, 11, 54, 13, 63, 15, 72, 17, 81, 19, 90, 21, 99, 23, 108, 25, 117, 27, 126, 29, 135, 31, 144, 33, 153, 35, 162, 37, 171, 39, 180, 41, 189, 43, 198, 45, 207, 47, 216, 49, 225, 51, 234, 53, 243, 55, 252, 57, 261, 59, 270, 61
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OFFSET
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0,3
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COMMENTS
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Partial sums give the generalized 13-gonal (or tridecagonal) numbers A195313.
a(n) is also the length of the n-th line segment of a rectangular spiral on the infinite square grid. The vertices of the spiral are the generalized 13-gonal numbers. - Omar E. Pol, Jul 27 2018
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LINKS
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FORMULA
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G.f.: x*(1+9*x+x^2)/((1-x)^2*(1+x)^2).
a(n) = (7*(-1)^n+11)*n/4.
Sum_{i=0..n} a(i) = A195313(n). (End)
Multiplicative with a(2^e) = 9*2^(e-1), a(p^e) = p^e for odd prime p. - Andrew Howroyd, Jul 23 2018
Dirichlet g.f.: zeta(s-1) * (1 + 7/2^s). - Amiram Eldar, Oct 25 2023
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MATHEMATICA
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With[{nn=30}, Riffle[9Range[0, nn], Range[1, 2nn+1, 2]]] (* Harvey P. Dale, Sep 24 2011 *)
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PROG
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(Magma) /* By definition */ &cat[[9*n, 2*n+1]: n in [0..33]]; // Bruno Berselli, Sep 16 2011
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CROSSREFS
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KEYWORD
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nonn,easy,mult
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AUTHOR
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STATUS
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approved
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