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A069017
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Triangular numbers of the form k^2 + k + 1.
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1
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1, 3, 21, 91, 703, 3081, 23871, 104653, 810901, 3555111, 27546753, 120769111, 935778691, 4102594653, 31788928731, 139367449081, 1079887798153, 4734390674091
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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FORMULA
| G.f.: (x^4+2x^3-16x^2+2x+1)/[(1-x)(1-6x+x^2)(1+6x+x^2)].
a(n)=34*a(n-2)-a(n-4)-11; a(n)=2*A124174(n)+1. [From Zak Seidov (zakseidov(AT)yahoo.com), Sep 25 2010]
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MAPLE
| Do[a = n(n + 1) + 1; b = Floor[Sqrt[2a]]; If[b(b + 1) == 2a, Print[a]], {n, 1, 106}]
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CROSSREFS
| Cf. A124174 [From Zak Seidov (zakseidov(AT)yahoo.com), Sep 25 2010]
Sequence in context: A129755 A059826 A108970 * A144883 A074597 A076207
Adjacent sequences: A069014 A069015 A069016 * A069018 A069019 A069020
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KEYWORD
| nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Apr 02 2002
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EXTENSIONS
| Program and terms from Robert G. Wilson v.
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