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A072521
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a(1) = 6 and then the smallest triangular numbers such that sum of two neighbors is also a triangular number.
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1
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6, 15, 21, 45, 91, 990, 1711, 365085, 401856, 713415, 785631, 1079715, 1326006, 2355535, 2888406, 5137615, 5666661, 5764710, 9550635, 9921285, 10934826, 19434495, 21421785, 23622501, 42003195, 46315500, 82349361, 146384605
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| The sequence is unbounded as a(n+1) is less than or equal to the n-th triangular number.
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EXAMPLE
| 45 is a term a 21 + 45 = 66 as well as 45 + 91 = 136 are triangular numbers.
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PROG
| (PARI) p=6:k=3:for(n=1, 30, k=k+1:u=p+k*(k+1)/2:t=floor(sqrt(2*u)):while(u!=t*(t+1)/2, k=k+1:u=p+k*(k+1)/2:t=floor(sqrt(2*u))):p=k*(k+1)/2:print1(p", "))
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CROSSREFS
| Cf. A072522.
Sequence in context: A015793 A063466 A138109 * A130178 A100410 A095032
Adjacent sequences: A072518 A072519 A072520 * A072522 A072523 A072524
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KEYWORD
| nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Jul 31 2002
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EXTENSIONS
| More terms from Ralf Stephan (ralf(AT)ark.in-berlin.de), Mar 30 2003
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