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Search: id:A128896
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| A128896 |
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Triangular numbers with exactly three distinct prime factors. |
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2
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| 66, 78, 105, 190, 231, 406, 435, 465, 561, 595, 741, 861, 903, 946, 1378, 1653, 2211, 2278, 2485, 3081, 3655, 3741, 4371, 4465, 5151, 5253, 5995, 6441, 7021, 7503, 8515, 8911, 9453, 9591, 10011, 10153, 10585, 11026, 12561, 13366, 14878, 15051, 15753
(list; graph; listen)
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OFFSET
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1,1
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FORMULA
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a(n)=T(k)=k(k+1)/2=p*q*r for some k,p,q,r, where T(k) is triangular number and p, q, r are distinct primes.
Equals A000217 INTERSECT A007304 and A075875 INTERSECT A121478. - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Apr 22 2007
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EXAMPLE
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a(1)=T(11)=66=2*3*11, a(2)=T(12)=78=2*3*13, a(3)=T(14)=105=3*5*7, a(4)=T(19)=190=2*5*19, a(5)=T(21)=231=3*7*11, a(6)=T(28)=406=2*7*29.
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MATHEMATICA
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Select[Table[n(n+1)/2, {n, 1, 210}], Transpose[FactorInteger[ # ]][[2]]=={1, 1, 1}&]
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CROSSREFS
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Cf. A000217, A068443, A069903, A076551, A127637.
Sequence in context: A039538 A095751 A121478 this_sequence A109750 A127654 A032485
Adjacent sequences: A128893 A128894 A128895 this_sequence A128897 A128898 A128899
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KEYWORD
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nonn
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AUTHOR
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Zak Seidov (zakseidov(AT)yahoo.com), Apr 20 2007
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