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Search: id:A128880
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| A128880 |
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Triangular numbers congruent to 1 or 5 mod 6. |
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+0
1
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| 1, 55, 91, 253, 325, 595, 703, 1081, 1225, 1711, 1891, 2485, 2701, 3403, 3655, 4465, 4753, 5671, 5995, 7021, 7381, 8515, 8911, 10153, 10585, 11935, 12403, 13861, 14365, 15931, 16471, 18145, 18721, 20503, 21115, 23005, 23653, 25651, 26335, 28441
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OFFSET
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1,2
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COMMENT
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Or, except for the first term, triangular numbers the least prime factor of which is >=5.
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FORMULA
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a(1)=Tr(1), a(2)=Tr(10), where Tr(k)=k(k+1)/2 is triangular number; for n>=3 a(n)=Tr(k(n)), where k(n)=k(n-2)+12 with k(1)=1, k(2)=10.
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MATHEMATICA
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c=0; Do[tr=n(n+1)/2; If[Abs[Mod[tr, 6]]==1, c++; a[c]=tr], {n, 300}]; Table[a[i], {i, c}]
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CROSSREFS
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Cf. A000217, A007310.
Sequence in context: A111192 A063873 A063131 this_sequence A039596 A115377 A146145
Adjacent sequences: A128877 A128878 A128879 this_sequence A128881 A128882 A128883
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KEYWORD
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nonn
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AUTHOR
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Zak Seidov (zakseidov(AT)gmail.com), Apr 18 2007, Apr 25 2007
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