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A214696
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Sum of next a(n) > 1 positive triangular numbers is a triangular number.
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2
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OFFSET
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1,1
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COMMENTS
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Two or more triangular numbers in the sum.
Because an oblong number is twice a triangular number, this sequence also gives: the sum of next a(n) positive oblong numbers is an oblong number.
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LINKS
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FORMULA
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a(n) is the smallest integer k > 1 such that (4*k^3 + 12*s*k^2 + 4*(3*s^2-1)*k)/3 + 1 is a square, where s = 1 + a(1) + a(2) + ... + a(n-1). - Max Alekseyev, Jan 30 2014
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EXAMPLE
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1+3+6 = 10 is a triangular number, so a(1)=3, then 10+15+21+28+36+45+55 = 210 is a triangular number, seven summands, so a(2)=7.
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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