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A240529
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Indices of 9-almost prime triangular numbers.
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2
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224, 351, 624, 704, 735, 768, 783, 800, 832, 864, 895, 944, 959, 960, 999, 1151, 1152, 1224, 1279, 1343, 1344, 1375, 1440, 1520, 1539, 1824, 1855, 1935, 1943, 1944, 1952, 2000, 2144, 2159, 2176, 2187, 2295, 2367, 2430, 2432, 2464, 2495, 2496, 2499, 2511
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OFFSET
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1,1
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 1..1000
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FORMULA
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{ m : A069904(m) = 9 }. - Alois P. Heinz, Aug 05 2019
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EXAMPLE
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a(1) = 224 because A000217(224) = 224*(224+1)/2 = 25200 = 2^4 * 3^2 * 5^2 * 7 is a 9-almost prime.
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MATHEMATICA
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Flatten[Position[Accumulate[Range[3500]], _?(PrimeOmega[#]== 9 &)]]
Select[Range[3000], PrimeOmega[(# (# + 1))/2] == 9 &] (* Harvey P. Dale, Jun 22 2017 *)
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PROG
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(Magma) [n: n in [2..2600] | &+[d[2]: d in Factorization((n*(n+1)))] eq 10]; // Vincenzo Librandi, Dec 22 2018
(GAP) F:=List([1..2600], n->Length(Factors(n*(n+1)/2)));; a:=Filtered([1..Length(F)], i->F[i]=9); # Muniru A Asiru, Dec 22 2018
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CROSSREFS
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Cf. A000217, A069904, A108815, A114435, A114436, A114437, A164977.
Sequence in context: A345543 A345796 A046296 * A177770 A202442 A331371
Adjacent sequences: A240526 A240527 A240528 * A240530 A240531 A240532
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KEYWORD
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nonn,easy
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AUTHOR
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Vincenzo Librandi, Apr 07 2014
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STATUS
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approved
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