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A129803
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Triangular numbers that are the sum of three consecutive triangular numbers.
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19
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10, 136, 1891, 26335, 366796, 5108806, 71156485, 991081981, 13803991246, 192264795460, 2677903145191, 37298379237211, 519499406175760, 7235693307223426, 100780206894952201, 1403687203222107385, 19550840638214551186, 272308081731781609216
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OFFSET
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1,1
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COMMENTS
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Indices m: 4, 16, 61, 229, 856, 3196, 11929, with recurrence m(i) = 5(m(i-1) - m(i-2)) + m(i-3) (see A133161).
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LINKS
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FORMULA
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a(n) = tr(m) = tr(k) + tr(k+1) + tr(k+2), where tr(k) = k(k+1)/2 = A000217(k).
a(n+2) = 14*a(n+1) - a(n) - 3.
a(n+1) = 7*a(n) - 3/2 + 1/2*sqrt(192*a(n)^2 - 96*a(n) - 15).
G.f.: x*(10-14*x+x^2) / ((1-x)*(1-14*x+x^2)). (End)
a(n) = (4-3*(7-4*sqrt(3))^n*(-2+sqrt(3))+3*(2+sqrt(3))*(7+4*sqrt(3))^n)/16. - Colin Barker, Mar 05 2016
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EXAMPLE
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With tr(k) = k(k+1)/2 = A000217(k):
10 = tr(4) = tr(1) + tr(2) + tr(3) = 1 + 3 + 6,
136 = tr(16) = tr(8) + tr(9) + tr(10) = 36 + 45 + 55,
1891 = tr(61) = tr(34) + tr(35) + tr(36) = 595 + 630 + 666,
26335 = tr(229) = tr(131) + tr(132) + tr(133) = 8646 + 8778 + 8911,
366796 = tr(856) = tr(493) + tr(494) + tr(495) = 121771 + 122265 + 122760.
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PROG
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(PARI) Vec((10*z - 14*z^2 + z^3)/((1-z)*(1 - 14*z + z^2)) + O(z^30)) \\ Michel Marcus, Sep 16 2015
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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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