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A121478
Triangular numbers with three distinct prime factors.
2
66, 78, 105, 120, 190, 231, 276, 300, 378, 406, 435, 465, 528, 561, 595, 666, 741, 820, 861, 903, 946, 1035, 1128, 1176, 1275, 1378, 1485, 1653, 1953, 2016, 2080, 2211, 2278, 2485, 2556, 2628, 2775, 3081, 3160, 3240, 3655, 3741, 3916, 4005, 4371, 4465
OFFSET
1,1
EXAMPLE
11*12/2 = 2*3*11 = 66 is the smalles triangular number with three distinct prime factors, hence a(1) = 66.
MATHEMATICA
Select[Accumulate[Range[0, 100]] , PrimeNu[#]==3&] (* James C. McMahon, Oct 19 2024 *)
PROG
(PARI) for(n=1, 100, k=binomial(n+1, 2); if(omega(k)==3, print1(k, ", ")))
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Klaus Brockhaus, Aug 01 2006
STATUS
approved