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A214648
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Sum of next a(n) > 1 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.
Sum of next a(n) oblong numbers is an oblong number: the same sequence.
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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 = a(1) + a(2) + ... + a(n-1). - Max Alekseyev, Jan 30 2014
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EXAMPLE
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(0+1)=1 is a triangular number, so a(1)=2, then (3+6+10+15+21)=55 is a triangular number, so a(2)=5.
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PROG
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(Python)
from __future__ import division
from gmpy2 import is_square
A214648_list, s, c, d = [], 0, 0, -1
for _ in range(10):
k = 2
q = 4*(k*(k*(k+c)+d))//3 + 1
while not is_square(q):
k += 1
q = 4*(k*(k*(k+c)+d))//3 + 1
s += k
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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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